Bluetooth: Host: Add NULL check for addr in unpair

unpair may be called from bt_unpair where addr is NULL.
One such case is the `bt clear all` shell command which
calls bt_unpair with addr = NULL, and the addr is just forwarded
to unpair which does not check for NULL.

bt_unpair allows for the addr to be NULL to clear all, but only
if SMP is enabled.

Modified the checks in bt_unpair to increase readability
and ensure that unpair is not called with NULL.

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