Don't add a WHERE if there are no constraints

lib/adapters/mysql.js

@@ -241,6 +241,9 @@ MySQL.prototype.all = function all(model, filter, callback) {
                 cs.push(keyEscaped + ' = ' + val);
             }
         });
+        if (cs.length === 0) {
+          return '';
+        }
         return 'WHERE ' + cs.join(' AND ');
     }