Bluetooth: BAP: Shell: Add tracking of empty SDUs

There are devices that will send empty SDUs instead of
empty PDUs, and while that should be harmless, it is nice
to know what is going on, and we may want to deal with
empty SDUs and empty PDUs differently.

This also modifies the state for TS and PSN to only be
applied for valid ISO packets.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>