Add support for nullif scalar function

testing/scalar-functions.test

@@ -279,6 +279,18 @@ do_execsql_test max-null {
   select max(null,null)
 } {}
 
+do_execsql_test nullif {
+  select nullif(1, 2)
+} {1}
+
+do_execsql_test nullif {
+  select nullif(1, 1)
+} {}
+
+do_execsql_test nullif {
+  select nullif('limbo', 'limbo')
+} {}
+
 do_execsql_test substr-3-args {
   SELECT substr('limbo', 1, 3);
 } {lim}