next_nonlocal_backend_entry_ is reset on empty Statestore messages

Currently processing updates from the Statestore roughly looks like this in the scheduler:

• if !is_delta: clear current information
• Process all new entries
• Remove all old entries
• Invalidate next_nonlocal_backend_entry_

However the invalidation of the next_nonlocal_backend_entry_ pointer happens unconditionally, whether there where changes to the current backend_map_ or not - including when an empty message was sent by the Statestore. This breaks most of the round-robin functionality in GetBackend(). As a fix, we should only reset next_nonlocal_backend_entry_ if there were changes to the backend map.