pgsql: Handle empty-string edge cases correctly in strpos().

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/43e43771bc4b03a9e33be7261cc84dc7f65ae9ef

Modified Files
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src/backend/utils/adt/varlena.c       | 10 +++++++++-
src/test/regress/expected/strings.out | 18 ++++++++++++++++++
src/test/regress/sql/strings.sql      |  6 ++++++
3 files changed, 33 insertions(+), 1 deletion(-)