Allow storing of Object.create(null) data in Firestore (#193)

Author: schmidt-sebastian

handwritten/firestore/src/document.js

@@ -1650,7 +1650,8 @@ function isPlainObject(input) {
   return (
     typeof input === 'object' &&
     input !== null &&
-    Object.getPrototypeOf(input) === Object.prototype
+    (Object.getPrototypeOf(input) === Object.prototype ||
+      Object.getPrototypeOf(input) === null)
   );
 }