As is, DBClientConnection takes in an applicationName here. But it never verifies that the applicationName is not empty. We should invariant that it isn't empty and change locations that were using empty applicationNames. This would allow us to explicitly see when shardsvr commands are on behalf of internal services like initial sync.

Acceptance criteria:

Change appNames to be non empty (e.g. (_FILE_) and log a warning when they are empty.