plugins: Fix for provisioning when there is no SPN

Android provisioning plugin was aborting when EF_SPN was not present in the SIM in case there was a MVNO for the SIM MCC+MNC pair.

Reason was that g_str_equal() is not NULL safe, so (g_str_equal(mvnomatch, apndb->match_spn) == FALSE) was aborting. The fix changes most g_str_equal calls to g_strcmp0, which is able to handle NULL pointers. g_str_equal has been kept in case we know that the pointer cannot be NULL, as it can have slightly better performance. g_str_equal is used normally as a hash table comparison function (see [1]).

[1] https://developer.gnome.org/glib/unstable/glib-Hash-Tables.html#g-str-equal