Code Modernization: Correct fallback return value in get_the_author().
If the $authordata global is not set, get_the_author() returned null, causing a PHP 8.1 "null to non-nullable" deprecation notice in ent2ncr() hooked via the_author filter:
str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
This commit updates get_the_author() to return an empty string if called before $authordata is set, bringing consistency with a few other similar functions which also return an empty string in this case:
get_the_author_meta()
get_the_author_posts_link()
get_the_modified_author()
Follow-up to [695/tests], [2858], [11138], [12284], [20575], [34677], [44616], [53187].
Props Soean, jrf, sabernhardt, salvoaranzulla, antpb, ebai4, sajjad67, tijmensmit, SergeyBiryukov.
Fixes #58157.
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