Add support for ifnull scalar function

testing/scalar-functions.test

@@ -19,6 +19,16 @@ do_execsql_test abs-null {
     select abs(null);
 } {}
 
+db null {}
+
+do_execsql_test ifnull-1 {
+  select ifnull(1, 2);
+} {1}
+
+do_execsql_test ifnull-2 {
+  select ifnull(null, 2);
+} {2}
+
 do_execsql_test upper {
   select upper('Limbo')
 } {LIMBO}