fix: return {} instead of null

Author: vdvibhu20

lib/baseController.js

@@ -110,9 +110,9 @@ class Controller {
   // cosntruct attributes clause based on
   generateAttributesClause (req) {
     const { fields } = req.query
-    if (!fields) return null
+    if (!fields) return {}
     if(!fields[this._type]) {
-      return null
+      return {}
     }
 
     return fields[this._type].split(',')