Fix issue 2201: unexpected empty string behavior (#2203)

This PR fixes the issue of some string functions returning NULL
instead of the empty string.

The string functions affected and corrected are -

reverse, toupper, tolower, rtrim, ltrim, trim, right, left, substring,
and replace.

Added additional regression tests. Corrected previous tests.

    modified:   regress/expected/expr.out
    modified:   regress/sql/expr.sql
    modified:   src/backend/utils/adt/agtype.c

Author: jrgemignani

regress/expected/expr.out

@@ -3927,6 +3927,41 @@ SELECT * FROM age_reverse('gnirts a si siht'::cstring);
  "this is a string"
 (1 row)
 
+-- should return empty string
+SELECT * FROM age_reverse('');
+ age_reverse 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_reverse(''::text);
+ age_reverse 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_reverse(''::cstring);
+ age_reverse 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN reverse('')
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN reverse("")
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
 -- should return null
 SELECT * FROM cypher('expr', $$
     RETURN reverse(null)
@@ -4104,6 +4139,75 @@ SELECT * FROM age_tolower('CSTRING'::cstring);
  "cstring"
 (1 row)
 
+-- should return empty string
+SELECT * FROM age_toupper('');
+ age_toupper 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_toupper(''::text);
+ age_toupper 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_toupper(''::cstring);
+ age_toupper 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN toupper('')
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN toupper("")
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
+SELECT * FROM age_tolower('');
+ age_tolower 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_tolower(''::text);
+ age_tolower 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_tolower(''::cstring);
+ age_tolower 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN tolower('')
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN tolower("")
+$$) AS (result agtype);
+ result 
+--------
+ ""
+(1 row)
+
 -- should return null
 SELECT * FROM cypher('expr', $$
     RETURN toUpper(null)
@@ -4211,6 +4315,73 @@ SELECT * FROM age_trim('  string   ');
  "string"
 (1 row)
 
+-- should return empty string
+SELECT * FROM cypher('expr', $$
+    RETURN lTrim('')
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN rTrim('')
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN trim('')
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN lTrim("")
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN rTrim("")
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM cypher('expr', $$
+    RETURN trim("")
+$$) AS (results agtype);
+ results 
+---------
+ ""
+(1 row)
+
+SELECT * FROM age_ltrim('');
+ age_ltrim 
+-----------
+ ""
+(1 row)
+
+SELECT * FROM age_rtrim('');
+ age_rtrim 
+-----------
+ ""
+(1 row)
+
+SELECT * FROM age_trim('');
+ age_trim 
+----------
+ ""
+(1 row)
+
 -- should return null
 SELECT * FROM cypher('expr', $$
     RETURN lTrim(null)
@@ -4322,15 +4493,16 @@ $$) AS (results agtype);
  "123"
 (1 row)
 
--- should return null
+-- should return empty string
 SELECT * FROM cypher('expr', $$
     RETURN left("123456789", 0)
 $$) AS (results agtype);
  results 
 ---------
- 
+ ""
 (1 row)
 
+-- should return null
 SELECT * FROM cypher('expr', $$
     RETURN left(null, 1)
 $$) AS (results agtype);
@@ -4401,15 +4573,16 @@ $$) AS (results agtype);
  "789"
 (1 row)
 
--- should return null
+-- should return empty string
 SELECT * FROM cypher('expr', $$
     RETURN right("123456789", 0)
 $$) AS (results agtype);
  results 
 ---------
- 
+ ""
 (1 row)
 
+-- should return null
 SELECT * FROM cypher('expr', $$
     RETURN right(null, 1)
 $$) AS (results agtype);
@@ -4508,6 +4681,13 @@ SELECT * FROM age_substring('0123456789', 1);
  "123456789"
 (1 row)
 
+-- should return empty string
+SELECT * FROM age_substring('0123456789', 0, 0);
+ age_substring 
+---------------
+ ""
+(1 row)
+
 -- should return null
 SELECT * FROM cypher('expr', $$
     RETURN substring(null, null, null)
@@ -4747,49 +4927,68 @@ $$) AS (results agtype);
  "ababab"
 (1 row)
 
--- should return null
+-- should return empty string
 SELECT * FROM cypher('expr', $$
-    RETURN replace(null, null, null)
+    RETURN replace("", "", "")
 $$) AS (results agtype);
  results 
 ---------
- 
+ ""
 (1 row)
 
 SELECT * FROM cypher('expr', $$
-    RETURN replace("Hello", null, null)
+    RETURN replace("Hello", "Hello", "")
 $$) AS (results agtype);
  results 
 ---------
- 
+ ""
 (1 row)
 
 SELECT * FROM cypher('expr', $$
-    RETURN replace("Hello", "", null)
+    RETURN replace("", "Hello", "Mellow")
 $$) AS (results agtype);
  results 
 ---------
- 
+ ""
+(1 row)
+
+SELECT * FROM age_replace('', '', '');
+ age_replace 
+-------------
+ ""
 (1 row)
 
+SELECT * FROM age_replace('Hello', 'Hello', '');
+ age_replace 
+-------------
+ ""
+(1 row)
+
+SELECT * FROM age_replace('', 'Hello', 'Mellow');
+ age_replace 
+-------------
+ ""
+(1 row)
+
+-- should return null
 SELECT * FROM cypher('expr', $$
-    RETURN replace("", "", "")
+    RETURN replace(null, null, null)
 $$) AS (results agtype);
  results 
 ---------
 
 (1 row)
 
 SELECT * FROM cypher('expr', $$
-    RETURN replace("Hello", "Hello", "")
+    RETURN replace("Hello", null, null)
 $$) AS (results agtype);
  results 
 ---------
 
 (1 row)
 
 SELECT * FROM cypher('expr', $$
-    RETURN replace("", "Hello", "Mellow")
+    RETURN replace("Hello", "", null)
 $$) AS (results agtype);
  results 
 ---------
@@ -4814,24 +5013,6 @@ SELECT * FROM age_replace('Hello', '', null);
 
 (1 row)
 
-SELECT * FROM age_replace('', '', '');
- age_replace 
--------------
- 
-(1 row)
-
-SELECT * FROM age_replace('Hello', 'Hello', '');
- age_replace 
--------------
- 
-(1 row)
-
-SELECT * FROM age_replace('', 'Hello', 'Mellow');
- age_replace 
--------------
- 
-(1 row)
-
 -- should fail
 SELECT * FROM cypher('expr', $$
     RETURN replace()