In some cases, code may look at the value of an array-valued property
during object initialization, before NMObject has set it to its actual
initial value. So ensure that we initialize all such properties to an
empty array, rather than leaving them NULL.

Also fix another bug in NMClient that could result in
priv->active_connections being NULL during certain signal emissions,
and fix nm_client_get_active_connections() to not return NULL when NM
was not running.