[#3325|Core] Fix unable to remove magnet with delete_copies enabled

Users were encountering the following error while attempting to delete
magnet torrents and had the config 'Delete copy of torrent file'
enabled. This was due to removing a magnet before the metadata was
downloaded and the torrent.filename was still set to None so raises
exceptions when string operations are performed with it.

  File "/usr/lib/python3/dist-packages/deluge/core/torrent.py", line 1317, in delete_torrentfile
    os.path.join(self.config['torrentfiles_location'], self.filename)
  ...
  TypeError: join() argument must be str or bytes, not 'NoneType'

Fixed by both setting a default empty string for self.filename and only
deleting the torrent file copy if filename is set.

deluge/core/torrent.py

@@ -266,6 +266,9 @@ class Torrent(object):
             self.is_finished = False
             self.filename = filename
 
+        if not self.filename:
+            self.filename = ''
+
         self.forced_error = None
         self.statusmsg = None
         self.state = None
@@ -1316,7 +1319,7 @@ class Torrent(object):
         torrent_files = [
             os.path.join(get_config_dir(), 'state', self.torrent_id + '.torrent')
         ]
-        if delete_copies:
+        if delete_copies and self.filename:
             torrent_files.append(
                 os.path.join(self.config['torrentfiles_location'], self.filename)
             )

deluge/tests/test_torrentmanager.py

@@ -56,6 +56,16 @@ class TorrentmanagerTestCase(BaseTestCase):
         )
         self.assertTrue(self.tm.remove(torrent_id, False))
 
+    @defer.inlineCallbacks
+    def test_remove_magnet(self):
+        """Test remove magnet before received metadata and delete_copies is True"""
+        magnet = 'magnet:?xt=urn:btih:ab570cdd5a17ea1b61e970bb72047de141bce173'
+        options = {}
+        self.core.config.config['copy_torrent_file'] = True
+        self.core.config.config['del_copy_torrent_file'] = True
+        torrent_id = yield self.core.add_torrent_magnet(magnet, options)
+        self.assertTrue(self.tm.remove(torrent_id, False))
+
     def test_prefetch_metadata(self):
         from deluge._libtorrent import lt