[firebase_database] FirebaseAnimatedList should have a 'emptyListChild'

[kroikie]

Current behavior

When using the firebaseAnimatedList for user generated content, the first time the user loads the application and there's no content, is just not optimal to load an empty container.

Expected behavior

The ability to show a custom widget when the list is empty

[kroikie]

@Panthro

The issue at flutter/flutter#15480 has been closed and moved here. Future collaboration on this issue will be done here.

[devroldy]

Any progress on this? There needs to be a custom widget for when FirebaseAnimatedList has no value in the list