Depending on the outcome of
CDRIVER-3012, we may want to raise an error if an empty string is received for the username URI option. This is assuming that there is no valid use case for authenticating against the server with an empty username.
If libmongoc ends up rejecting an empty username during URI parsing and perhaps also in mongoc_uri_set_username(), this may not require any work in the PHP driver. Alternatively, libmongoc may decide that an empty username is equivalent to a null username in terms of neither requiring authentication during mongoc_cluster_init().