fix: Make rtdb `ref.push()` only create local node when no value is passed

[jonathanedey]

The optional value argument in push() can be used to provide an initial value for the child node. If no value is provided, child node will have an empty string as the default value.

This behaviour was inconsistent with the client SDKs. If no value is provided, push() should now return a reference that can be used elsewhere and should not create an empty node on the server.