tmf: Fix mouse actions disabled when no visible events in histogram

tmf: Fix mouse actions disabled when no visible events in histogram

Change conditions from zero events to empty time window to detect unused
histogram and disable mouse actions.

Also removes some unnecessary checks.

Change-Id: I761473005d6b004101741c535e8ec98a2bc0870f
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/19324
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>