fix empty string handling for ASCII SQL function

Author: pierreforstmann

contrib/ivorysql_ora/expected/ora_ascii.out

@@ -50,3 +50,9 @@ select ascii(to_char('2026-01-22 01:02:03.00 +01:00'::timestamptz,'DD-MON-YYYY H
     50
 (1 row)
 
+select ascii('') is null from dual;
+ ?column? 
+----------
+ t
+(1 row)
+

contrib/ivorysql_ora/sql/ora_ascii.sql

@@ -17,3 +17,5 @@ select ascii(to_char('2026-01-01'::date,'DD-MON-YYYY HH24:MI:SS')) from dual;
 select ascii(to_char('2026-01-11 01:02:03.00'::timestamp,'DD-MON-YYYY HH24:MI:SS.FF6')) from dual;
 
 select ascii(to_char('2026-01-22 01:02:03.00 +01:00'::timestamptz,'DD-MON-YYYY HH24:MI:SS TZH:TZM')) from dual;
+
+select ascii('') is null from dual;

contrib/ivorysql_ora/src/builtin_functions/character_datatype_functions.c

@@ -2431,8 +2431,13 @@ ora_ascii(PG_FUNCTION_ARGS)
 	 	}
     }
 
-    if (str == NULL || str[0] == '\0')
-        PG_RETURN_INT32(0);
+    if (str == NULL || str[0] == '\0') {
+		/* 1. Set the null flag */
+		fcinfo->isnull = true;
+
+		/* 2. Return a placeholder value */
+		PG_RETURN_VOID();
+	}
 
     PG_RETURN_INT32((unsigned char) str[0]);
 }