Fix NPE when trying to clear a database value, i.e. set it to null

Author: chrisbianca

lib/modules/database/reference.js

@@ -155,6 +155,9 @@ export default class Reference extends ReferenceBase {
   // Sanitize value
   // As Firebase cannot store date objects.
   _serializeValue(obj: Object = {}) {
+    if (!obj) {
+      return obj;
+    }
     return Object.keys(obj).reduce((sum, key) => {
       let val = obj[key];
       if (val instanceof Date) {