[database] fix flow issue

lib/modules/database/DataSnapshot.js

@@ -62,7 +62,9 @@ export default class DataSnapshot {
       // todo this is wrong - child keys needs to be the ordered keys, from FB
       // todo potential solution is build up a tree/map of a snapshot and its children
       // todo natively and send that back to JS to be use in this class.
-      childKeys: isObject(value) ? Object.keys(value) : [],
+
+      // null check to keep flow happy even though isObject already does this
+      childKeys: isObject(value) && value !== null ? Object.keys(value) : [],
     });
   }