Blender has a limit for both vertex color layers and UV layers. The
functions `bpy.types.Mesh.vertex_colors.new()` and
`bpy.types.Mesh.uv_layers.new()` will return `None` once the limit
is reached. The FBX importer and glTF importer didn't handle this
case before and attempted to access the `data`, which failed. This
patch adds the missing checks. In case no vertex colors or uv map
can be created, the assignment of colors or uv coordinates is
skipped.
Reviewed By: mont29, julien
Differential Revision: https://developer.blender.org/D9613
