Bluetooth: GATT: Fix segfault discover descriptors all UUIDs

Handle optional argument UUID in bt_gatt_discover with type
DISCOVER_DESCRIPTOR, bt_uuid_cmp doesn't check for NULL pointer.
On system with MMU (nrf52_bsim) this can result in segfault.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>