[epg] don't use epg tag that was active as a currently active one if epg contains gaps

The current behaviour is to show previous programme in PVR channels and EPG:Now views if there's a gap in channel EPG now. This encourages user to click on channel but nothing will be played (e.g. PVR client may not provide a channel stream).
This commit changes the behaviour so nothing will be displayed under channel and in EPG:Now view in case if there's no current programme.