Handle empty-string edge cases correctly in strpos().
Commit
9556aa01c rearranged the innards of text_position() in a way
that would make it not work for empty search strings. Which is fine,
because all callers of that code special-case an empty pattern in
some way. However, the primary use-case (text_position itself) got
special-cased incorrectly: historically it's returned 1 not 0 for
an empty search string. Restore the historical behavior.
Per complaint from Austin Drenski (via Shay Rojansky).
Back-patch to v12 where it got broken.
Discussion: https://postgr.es/m/CADT4RqAz7oN4vkPir86Kg1_mQBmBxCp-L_=9vRpgSNPJf0KRkw@mail.gmail.com