fix: use cardinality tests for string_to_array empty input and add missing test case

Use cardinality() to unambiguously verify empty arrays since Arrow's
text format renders [""] identically to []. Add missing test for
string_to_array('', '', 'x') covering all four is_empty checks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dd-david-levin

datafusion/sqllogictest/test_files/array.slt

@@ -8623,21 +8623,25 @@ SELECT string_to_array('abcxxxdef', 'xxx')
 ----
 [abc, def]
 
-# empty string input should return empty array (PostgreSQL compatibility)
-query ?
-SELECT string_to_array('', ',')
+query I
+SELECT cardinality(string_to_array('', ','))
 ----
-[]
+0
 
-query ?
-SELECT string_to_array('', '')
+query I
+SELECT cardinality(string_to_array('', ''))
 ----
-[]
+0
 
-query ?
-SELECT string_to_array('', ',', 'x')
+query I
+SELECT cardinality(string_to_array('', ',', 'x'))
 ----
-[]
+0
+
+query I
+SELECT cardinality(string_to_array('', '', 'x'))
+----
+0
 
 query ?
 SELECT string_to_array('abc', '')