[Tools] Fix mono-api-html MarkdownFormatter.cs to avoid a NRE (#21430)

One of the changes in macios revealed a bug in which @new can be null
resulting in a NRE like the one that follows:

System.NullReferenceException: Object reference not set to an instance of an object
2022-01-26T09:23:11.9538790Z   at Mono.ApiTools.MarkdownFormatter.DiffModification (System.Text.StringBuilder output, System.String old, System.String new, System.Boolean breaking) [0x00013] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/MarkdownFormatter.cs:181
2022-01-26T09:23:11.9543530Z   at Mono.ApiTools.ApiChange.AppendModified (System.String old, System.String new, System.Boolean breaking) [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ApiChange.cs:49
2022-01-26T09:23:11.9547700Z   at Mono.ApiTools.FieldComparer.Equals (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target, Mono.ApiTools.ApiChanges changes) [0x0008c] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/FieldComparer.cs:123
2022-01-26T09:23:11.9551430Z   at Mono.ApiTools.MemberComparer.Compare (System.Collections.Generic.IEnumerable`1[T] source, System.Collections.Generic.IEnumerable`1[T] target) [0x00053] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/MemberComparer.cs:111
2022-01-26T09:23:11.9554840Z   at Mono.ApiTools.MemberComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x00064] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/MemberComparer.cs:66
2022-01-26T09:23:11.9558230Z   at Mono.ApiTools.ClassComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target, Mono.ApiTools.ApiChanges diff) [0x00172] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ClassComparer.cs:238
2022-01-26T09:23:11.9562840Z   at Mono.ApiTools.Comparer.Compare (System.Collections.Generic.IEnumerable`1[T] source, System.Collections.Generic.IEnumerable`1[T] target) [0x00084] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/Comparer.cs:94
2022-01-26T09:23:11.9566810Z   at Mono.ApiTools.ClassComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x0002c] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ClassComparer.cs:68
2022-01-26T09:23:11.9570610Z   at Mono.ApiTools.NamespaceComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target, Mono.ApiTools.ApiChanges differences) [0x00017] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/NamespaceComparer.cs:79
2022-01-26T09:23:11.9574480Z   at Mono.ApiTools.Comparer.Compare (System.Collections.Generic.IEnumerable`1[T] source, System.Collections.Generic.IEnumerable`1[T] target) [0x00084] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/Comparer.cs:94
2022-01-26T09:23:11.9578000Z   at Mono.ApiTools.NamespaceComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x0002c] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/NamespaceComparer.cs:51
2022-01-26T09:23:11.9581650Z   at Mono.ApiTools.AssemblyComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target, Mono.ApiTools.ApiChanges diff) [0x00055] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/AssemblyComparer.cs:89
2022-01-26T09:23:11.9586220Z   at Mono.ApiTools.Comparer.Compare (System.Collections.Generic.IEnumerable`1[T] source, System.Collections.Generic.IEnumerable`1[T] target) [0x00084] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/Comparer.cs:94
2022-01-26T09:23:11.9589520Z   at Mono.ApiTools.AssemblyComparer.Compare () [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/AssemblyComparer.cs:63
2022-01-26T09:23:11.9592880Z   at Mono.ApiTools.ApiDiffFormatted.Generate (Mono.ApiTools.AssemblyComparer ac, System.IO.TextWriter outStream, Mono.ApiTools.State state) [0x00013] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ApiDiff.cs:222
2022-01-26T09:23:11.9596490Z   at Mono.ApiTools.ApiDiffFormatted.Generate (System.String firstInfo, System.String secondInfo, System.IO.TextWriter outStream, Mono.ApiTools.State state) [0x00008] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ApiDiff.cs:208
2022-01-26T09:23:11.9600250Z   at Mono.ApiTools.ApiDiffFormatted.Generate (System.String firstInfo, System.String secondInfo, System.IO.TextWriter outStream, Mono.ApiTools.ApiDiffFormattedConfig config) [0x00007] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ApiDiff.cs:202
2022-01-26T09:23:11.9603590Z   at Mono.ApiTools.Program.Main (System.String[] args) [0x002f9] in /Users/builder/jenkins/workspace/archive-mono/2020-02/ios/release/mcs/tools/mono-api-html/ApiDiff.cs:159

The change that exposed the bug is: https://github.com/xamarin/xamarin-macios/commit/d78aa6712b15659b0fed259952364c060ffedcea

The above exception is avoided by ensuring that nor old or @new are
null.

1 files changed, 2 insertions, 2 deletions

diff --git a/mcs/tools/mono-api-html/MarkdownFormatter.cs b/mcs/tools/mono-api-html/MarkdownFormatter.cs
index 978b805532c..d2bbddaa03f 100644
--- a/mcs/tools/mono-api-html/MarkdownFormatter.cs
+++ b/mcs/tools/mono-api-html/MarkdownFormatter.cs
@@ -176,9 +176,9 @@ namespace Mono.ApiTools {
 
 		public override void DiffModification (StringBuilder output, string old, string @new, bool breaking)
 		{
-			if (old.Length > 0)
+			if (old != null && old.Length > 0)
 				DiffAddition (output, old, breaking);
-			if (@new.Length > 0)
+			if (@new != null && @new.Length > 0)
 				DiffRemoval (output, @new, true);
 		}