Don't bail out of term_exists() when term sanitizes to an empty string.

This change brings return values for failures of this sort in line with other
failed lookups in term_exists(): a null value is now returned in all cases
where the queried term is not found.

Adds unit test for the sanitization issue. Modifies existing unit test to
reflect the change in return value for empty term strings.

Props boonebgorges, georgestephanis.
Fixes #29589.

git-svn-id: https://develop.svn.wordpress.org/trunk@29865 602fd350-edb4-49c9-b593-d223f7449a82