[release/6.0] Fix nullable annotations for AssociatedMetadataTypeTypeDescriptionProvider

[github-actions]

Backport of #57932 to release/6.0

/cc @stephentoub

Customer Impact

Minor APIs weren't annotated as part of the TypeConverter annotations. This adds those annotations.

Testing

It compiles.

Risk

Minimal. It's purely annotation, no functional changes.

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Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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Issue Details

---

Backport of #57932 to release/6.0

/cc @stephentoub

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.ComponentModel.DataAnnotations, new-api-needs-documentation
Milestone:	-

[stephentoub]

@jeffhandley, I went ahead and asked the bot to backport this as part of finishing netcoreapp annotations. But feel free to close this if you don't think it's warranted.

cc: @danmoseley

[danmoseley]

approved for release/6.0 as negligible risk with some visible customer benefit. we'd probably reject this a bit later.
thanks for doing it.