Documentation of `get_preview_post_link` declares missing return variable type information

[chris_dev]

Super simple, but I just noticed that the inline documentation for the get_preview_post_link says it returns string but actually it returns void as well.

Trivial patch attached.

[swissspidy]

It needs to be documented in which cases it returns void.

Though I think it's weird it doesn't return an empty string or false. Perhaps changing that is the better solution.

[chris_dev]

@swissspidy Hi, I attached another patch file. Is it what you meant ?

[swissspidy]

@chris_dev Yes, that's what I meant. Thanks for the refreshed patch!

I added some unit tests in 35915.diff​ and changed the return value to null because that's what a void function returns in PHP.

[DrewAPicture]

35915.diff​ needs a refresh.

[DrewAPicture]

In 37564:

Docs: Update the return description for get_preview_post_link() to note that it can also return null.

Props chris_dev, swissspidy.
See #35915.

[DrewAPicture]

In 37566:

Posts: Add test coverage for get_preview_post_link().

Props swissspidy.
Fixes #35915.