GDBus: Properly handle empty address strings

GDBus: Properly handle empty address strings

Changes this error

 DBUS_SESSION_BUS_ADDRESS= \
 gdbus introspect --session \
                  --dest org.freedesktop.DBus \
                  --object-path /org/freedesktop/DBus
 **
 GLib-GIO:ERROR:gdbusaddress.c:913:g_dbus_address_get_stream_sync: assertion failed: (last_error != NULL)
 Aborted (core dumped)

to

 DBUS_SESSION_BUS_ADDRESS= \
 gdbus introspect --session \
                  --dest org.freedesktop.DBus \
                  --object-path /org/freedesktop/DBus
 Error connecting: The given address is empty

which is much more preferable.

Signed-off-by: David Zeuthen <davidz@redhat.com>