Bluetooth: BAP: Add missing ep check in client_qos

The bt_bap_unicast_client_qos function was recently
modified to check for presentation delay, however it did
not consider that a unicast group may contain streams that
are not configured, and thus do not have endpoints.

Add a stream->ep != NULL check to only consider configured
streams.

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