wifi: ensure not passing NULL bssid as string to printf logging functions

Signed-off-by: Thomas Haller <thaller@redhat.com>
wifi: avoid assertion in nm_ap_check_compatible() when comparing AP with missing BSSID

Signed-off-by: Thomas Haller <thaller@redhat.com>
wifi: Empty AP BSSID is NULL, not an invalid address

Since 3a54d050 the AP address is not a gbyte[], but a char *. The fake AP BSSID
fixup could trigger an assertion failure:

Oct 26 11:14:45 goatlord.localdomain NetworkManager[540]: nm_ethernet_address_is_valid: assertion 'addr != NULL' failed

https://bugzilla.gnome.org/show_bug.cgi?id=739258

Fixes: 3a54d050985d6ef2067b025571910a8ccd3cbd57
libnm-util: fix typos in libnm-util documentation
merge: wifi bssid handling fixes

Avoid passing NULL bssid where it does not make sense, fix a couple of bad
asserts.

https://bugzilla.gnome.org/show_bug.cgi?id=739258