Fixes previous commit to avoid checking for sql_comparator on undefined

Author: dresende

lib/Drivers/DML/mongodb.js

@@ -183,7 +183,7 @@ function convertToDB(obj) {
 			continue;
 		}
 
-		if (typeof obj[k].sql_comparator == "function") {
+		if (obj[k] && typeof obj[k].sql_comparator == "function") {
 			var val       = (k != "_id" ? obj[k].val : new mongodb.ObjectID(obj[k].val));
 			var comp      = obj[k].sql_comparator();
 			var condition = {};