Depsgraph: skip parentbone relation if bone prop is empty

Clearing the Parent Bone field in relations would result in something
like this:

```
add_relation(Bone Parent) - Could not find op_from
(ComponentKey(OBArmature, BONE))
add_relation(Bone Parent) - Failed, but op_to (ComponentKey(OBEmpty,
TRANSFORM)) was ok
ERROR (bke.object): /source/blender/blenkernel/intern\object.c:3330
ob_parbone: Object Empty with Bone parent: bone  doesn't exist
```

Now skip creation of a depsgraph relation if the Parent Bone field is
empty (since this would be invalid anyways).

ref. T91101

Maniphest Tasks: T91101

Differential Revision: https://developer.blender.org/D12389

1 files changed, 7 insertions, 5 deletions

diff --git a/source/blender/depsgraph/intern/builder/deg_builder_relations.cc b/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
index ef1db8be933..28cfc5a9e1a 100644
--- a/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
+++ b/source/blender/depsgraph/intern/builder/deg_builder_relations.cc
@@ -1006,11 +1006,13 @@ void DepsgraphRelationBuilder::build_object_parent(Object *object)
 
     /* Bone Parent */
     case PARBONE: {
-      ComponentKey parent_bone_key(parent_id, NodeType::BONE, object->parsubstr);
-      OperationKey parent_transform_key(
-          parent_id, NodeType::TRANSFORM, OperationCode::TRANSFORM_FINAL);
-      add_relation(parent_bone_key, object_transform_key, "Bone Parent");
-      add_relation(parent_transform_key, object_transform_key, "Armature Parent");
+      if (object->parsubstr[0] != '\0') {
+        ComponentKey parent_bone_key(parent_id, NodeType::BONE, object->parsubstr);
+        OperationKey parent_transform_key(
+            parent_id, NodeType::TRANSFORM, OperationCode::TRANSFORM_FINAL);
+        add_relation(parent_bone_key, object_transform_key, "Bone Parent");
+        add_relation(parent_transform_key, object_transform_key, "Armature Parent");
+      }
       break;
     }