GDBusProxy: if a well-known name is not owned, don't GetAll from the dbus-daemon

GDBusProxy: if a well-known name is not owned, don't GetAll from the dbus-daemon

If you run:

    ( cd gio/tests && G_DBUS_DEBUG=all ./gdbus-proxy-well-known-name )

you can see that in the case where the name com.example.TestService isn't
owned yet, the GDBusProxy calls GetAll() with no destination, resulting
in an error reply from the peer (the dbus-daemon itself). That's clearly
not right!

However, if priv->name is NULL, that indicates the special case where we
really do want to talk directly to a peer, instead of via the bus daemon
(most likely to be used on peer-to-peer connections); in that special
case, do call GetAll().

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: David Zeuthen <davidz@redhat.com>