Bluetooth: Audio: Fix broadcast assistant recv state pointer value

When reporting a receive state to the upper layers, the broadcast
assistant should report NULL if there is not a receive state
active. The "activeness" of a receive state is determined by
the length of the characteristic - If it is 0 then there
is no receive state active.

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