Do not return $clusterTime when no keys are available

The problem

There are occasional errors while starting up mongodb replica set detailed in SERVER-31916.
The question is if the change similar to the shell fix to wait for the clusterTime signature should be implemented in all drivers or if its possible to fix in the server.
On the server side this call
https://github.com/mongodb/mongo/blob/r3.7.2/src/mongo/db/service_entry_point_mongod.cpp#L289
returns the dummy signature from mongod if the keys are not there yet. Its necessary for the proper replica set start up.

The solution

Its ok to not return $clusterTime if there are no keys available. The internal clients always use the internal user to communicate to each other - its a different code path. Returning no signature to external clients only in the case when auth is on will fix problems in SERVER-31916