Fix misleading message to user in certain "switch" scenarios.
Fix misleading message to user in certain "switch" scenarios.

If:

* time log is inactive; and
* user uses `switch` command with `--at` option and blank activity

then:

* the application was reporting a successful switch to "inactivity"
  (even though it was, correctly, not making any entry into the log).

This commit makes it instead report, in those circumstances, that
the log is already inactive.

matt-harvey/swx,skybaboon/swx