Improve test

Author: kevinmingtarja

datafusion/sqllogictest/test_files/functions.slt

@@ -1029,7 +1029,8 @@ SELECT
 query TIT
 SELECT str, n, substring_index(str, '.', n) AS c FROM
   (VALUES
-    ROW(NULL)
+    ROW(NULL),
+    ROW('abcd')
   ) AS strings(str),
   (VALUES
     ROW(1),
@@ -1039,6 +1040,10 @@ SELECT str, n, substring_index(str, '.', n) AS c FROM
   ) AS occurrences(n)
 ORDER BY str, n;
 ----
+abcd -2 abcd
+abcd -1 abcd
+abcd 1 abcd
+abcd 2 abcd
 NULL -2 NULL
 NULL -1 NULL
 NULL  1 NULL