Throw error when setting authData to null (#6154)

* added ignore authData field

* add fix for Postgres

* add test for mongoDB

* add test login with provider despite invalid authData

* removed fit

* fixed ignoring authData in postgres

* Fix postgres test

* Throw error instead of ignore

* improve tests

* Add mongo test

* allow authData when not user class

* fix tests

* more tests

* add condition to synthesize authData field only in _User class

it is forbidden to add a custom field name beginning with `_`, so if the object is not `_User` , the transform should throw

* add warning log when ignoring invalid `authData` in `_User`

* add test to throw when custom field begins with underscore

src/RestWrite.js

@@ -417,8 +417,21 @@ RestWrite.prototype.validateAuthData = function() {
     }
   }
 
-  if (!this.data.authData || !Object.keys(this.data.authData).length) {
+  if (
+    (this.data.authData && !Object.keys(this.data.authData).length) ||
+    !Object.prototype.hasOwnProperty.call(this.data, 'authData')
+  ) {
+    // Handle saving authData to {} or if authData doesn't exist
     return;
+  } else if (
+    Object.prototype.hasOwnProperty.call(this.data, 'authData') &&
+    !this.data.authData
+  ) {
+    // Handle saving authData to null
+    throw new Parse.Error(
+      Parse.Error.UNSUPPORTED_SERVICE,
+      'This authentication method is unsupported.'
+    );
   }
 
   var authData = this.data.authData;