apparmor: allocate xmatch for nullpdb inside aa_alloc_null

attach->xmatch was not set when allocating a null profile, which is used in
complain mode to allocate a learning profile. This was causing downstream
failures in find_attach, which expected a valid xmatch but did not find
one under a certain sequence of profile transitions in complain mode.

This patch ensures the xmatch is set up properly for null profiles.

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>