Bluetooth: Audio: Add check when registering media_proxy_pl_calls

Add NULL checks when registering the media_proxy_pl_calls.
Currently the media player expects all of them to be set.

In the future, ideally the callbacks would mostly be
optional and reflect which of the optional
characteristics that there is in the MCS.

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