Correctly handle empty dive list after delete

Things need to be cleared out and disabled. But fixing this bug leaves
another issue - while the newly selected dive after a delete acts as if it
was selected, it doesn't show up as selected in the dive list.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>