[WIP debug this closely][FIXUP] Fix subid parsing in member expression

Author: mcheshkov

packages/cubejs-api-gateway/src/query.js

@@ -109,7 +109,7 @@ const inputMemberExpression = Joi.object().keys({
   groupingSet: Joi.object().keys({
     groupType: Joi.valid('Rollup', 'Cube').required(),
     id: Joi.number().required(),
-    subId: Joi.number(),
+    subId: Joi.number().allow(null),
   }).allow(null)
 });