services/presence/server_plugin.py: Ensure _conn always exists (even if None)

This means cleanup() will not fail on an uninitialized server plugin,
and get_connection() will return None rather than raising AttributeError.

services/presence/server_plugin.py

@@ -125,6 +125,7 @@ class ServerPlugin(gobject.GObject):
         """
         gobject.GObject.__init__(self)
 
+        self._conn = None
         self._icon_cache = BuddyIconCache()
 
         self._registry = registry