Annotate valuePublishedByPlugin with nullable

[ened]

Description

Adds the missing nullable flag to [PluginRegistry valuePublishedByPlugin].

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.

[cbracken]

Thank you for your contribution -- lgtm!

[googlebot]

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We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[googlebot]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[cbracken]

@googlebot I consent.

[googlebot]

CLAs look good, thanks!

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[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[cbracken]

I've rebased your patch to tip-of-tree. The trybot was repeatedly failing on one of the scenarios tests in a way that looks unrelated to your patch. Hopefully this gets a clean presubmit run.

Apologies for the delay in getting to this.