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

src/test/regress/expected/strings.out

@@ -1371,6 +1371,24 @@ SELECT strpos('abcdef', 'xy') AS "pos_0";
      0
 (1 row)
 
+SELECT strpos('abcdef', '') AS "pos_1";
+ pos_1 
+-------
+     1
+(1 row)
+
+SELECT strpos('', 'xy') AS "pos_0";
+ pos_0 
+-------
+     0
+(1 row)
+
+SELECT strpos('', '') AS "pos_1";
+ pos_1 
+-------
+     1
+(1 row)
+
 --
 -- test replace
 --

src/test/regress/sql/strings.sql

@@ -479,6 +479,12 @@ SELECT strpos('abcdef', 'cd') AS "pos_3";
 
 SELECT strpos('abcdef', 'xy') AS "pos_0";
 
+SELECT strpos('abcdef', '') AS "pos_1";
+
+SELECT strpos('', 'xy') AS "pos_0";
+
+SELECT strpos('', '') AS "pos_1";
+
 --
 -- test replace
 --