The check was checking for an empty string in `matrix_jitsi_prosody_auth_internal_accounts`,
which is unlikely to happen. We should check for an empty list instead.
The check was not validating username/password values, so telling the user that they need a non-empty
username/password is misleading. It was merely checking if there's at least one entry in the list.
This patch adjusts the check and message accordingly.
6568b68759