Prevent empty author links in list tables

[snehapatil2001]

Trac ticket: https://core.trac.wordpress.org/ticket/62913

Description

This PR fixes the issue where empty author links appear in the media list table when no author is assigned. Currently, an empty <a> element is rendered which causes issues with keyboard navigation and accessibility tools.

Changes proposed in this Pull Request:

• Added empty author check before rendering author links
• Added fallback display with em dash (—) for visual indication
• Added screen reader text "(no author)" for better accessibility
• Added unit tests to verify both empty and valid author scenarios
• Maintains consistency with how empty values are handled in other columns

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props snehapatil02, audrasjb, sabernhardt.

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[github-actions]

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[sabernhardt]

In addition to updating the general post tables author column, you could make a few more edits for the media list tables in this PR.

I included suggestions for editing the unit tests, but I think removing the tests would be better than trying to fix them. Somehow the test for a valid author gets the no-author HTML, which makes the other test's success questionable.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60032
GitHub commit: d6e3b8a

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.