Revisiting nullable annotations on caching

[maryamariyan]

This PR fixes some mismatches between nullability of the arguments between ref and src

[ghost]

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.

[ghost]

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Issue Details

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Using GenerateReferenceAssemblySource target on caching libraries I noticed a mismatch between nullability of the arguments between ref and src for Microsoft.Extensions.Caching.Abstraction for CacheExtensions.GetOrCreateAsync and for PostEvictionDelegate.

@eerhardt @maxkoshevoi could you please review?

Author:	maryamariyan
Assignees:	maryamariyan
Labels:	new-api-needs-documentation, area-Extensions-Caching
Milestone:	-

[maryamariyan]

Updated this PR, some of the changes were already addressed in #65804

[tarekgh]

@maryamariyan I am seeing this PR changes almost no-op. am I missing anything here?

[maryamariyan]

PR is ready for review.

@tarekgh aside from changes requested by @eerhardt in #65775 I am also fixing the placement of Clear in caching ref file, as dictated via GenerateReferenceAssemblySource target.

[eerhardt]

LGTM. Thanks!

[maxkoshevoi]

LGTM