Don't deliver empty resume_data parameter to session.add_torrent

Fixes a libtorrent crash on python 2.7 on Windows

deluge/core/torrentmanager.py

@@ -383,6 +383,7 @@ class TorrentManager(component.Component):
                 resume_data = self.legacy_get_resume_data_from_file(state.torrent_id)
                 self.legacy_delete_resume_data(state.torrent_id)
 
+            if resume_data:
                 add_torrent_params["resume_data"] = resume_data
         else:
             # We have a torrent_info object or magnet uri so we're not loading from state.