don't include empty sessions in _chain_lengths method

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

slixmpp_omemo/__init__.py

@@ -532,6 +532,8 @@ class XEP_0384(BasePlugin):
         lengths: ChainLengths = {'sending': [], 'receiving': []}
         for did in devices:
             session = self._omemo._SessionManager__loadSession(bare, did)
+            if session is None:
+                break
             skr = session._DoubleRatchet__skr
             lengths['sending'].append((did, skr.sending_chain_length))
             lengths['receiving'].append((did, skr.receiving_chain_length))