Consider changing TypeDescriptor.GetTypeDescriptor to return EmptyCustomTypeDescriptor instead of null

[krwq]

Found during adding nullable annotations to TypeConverter (#54961)

Currently TypeDescriptor.GetTypeDescriptor returns null when instance is null.

Consider changing that logic to return EmptyCustomTypeDescriptor and changing nullability of return type.

Current logic causes that GetTypeDescriptor needs to return nullable ICustomTypeDescriptor which causes that base class needs to be annotated the same which adds lots of unnecessary !. Returning EmptyCustomTypeDescriptor seems more consistent with other implementations though and also seems no code in the framework expects that API to return null.

Please make sure to track usages and remove unnecessary ! along the way when fixing this

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Issue Details

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Found during adding nullable annotations to TypeConverter (#54961)

Currently TypeDescriptor.GetTypeDescriptor returns null when instance is null.

Consider changing that logic to return EmptyCustomTypeDescriptor and changing nullability of return type.

Current logic causes that GetTypeDescriptor needs to return nullable ICustomTypeDescriptor which causes that base class needs to be annotated the same which adds lots of unnecessary !. Returning EmptyCustomTypeDescriptor seems more consistent with other implementations though and also seems no code in the framework expects that API to return null.

Author:	krwq
Assignees:	-
Labels:	area-System.ComponentModel
Milestone:	-