Review usage of nullable suppressions (especially `!`) in CodeExporter.cs

[ViktorHofer]

#78703 adds a couple of nullable suppressions that should be removed and cleaned-up. Those suppressions are necessary as previously, the assembly compiled against ref/System.CodeDom but with the above PR now binds against src/System.CodeDom which has a different nullability setting.

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Issue Details

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#78703 adds a couple of nullable suppressions that should be removed and cleaned-up. Those suppressions are necessary as previously, the assembly compiled against ref/System.CodeDom but with the above PR now binds against src/System.CodeDom which has a different nullability setting.

Author:	ViktorHofer
Assignees:	-
Labels:	area-Meta
Milestone:	8.0.0

[halgab]

Winforms repo has problems as well with the current state of nullability annotations in System.CodeDom. We bumped into the same problems with CodeFieldReferenceExpression, CodePrimitiveExpression and CodePropertyReferenceExpression. See dotnet/winforms#9663