Don't fail on decrypting heartbeats

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

slixmpp_omemo/__init__.py

@@ -650,9 +650,10 @@ class XEP_0384(BasePlugin):
         allow_untrusted: bool = False,
     ) -> Optional[str]:
         header = encrypted['header']
-        if encrypted['payload']['value'] is None:
-            raise ErroneousPayload('The payload element was empty')
-        payload = b64dec(encrypted['payload']['value'])
+
+        payload = None
+        if encrypted['payload']['value'] is not None:
+            payload = b64dec(encrypted['payload']['value'])
 
         jid = sender.bare
         sid = int(header['sid'])
@@ -674,15 +675,26 @@ class XEP_0384(BasePlugin):
         # XXX: 'cipher' is part of KeyTransportMessages and is used when no payload
         # is passed. We do not implement this yet.
         try:
-            body = self._omemo.decryptMessage(
-                jid,
-                sid,
-                iv,
-                message,
-                isPrekeyMessage,
-                payload,
-                allow_untrusted=allow_untrusted,
-            )
+            if payload is None:
+                self._omemo.decryptRatchetFowardingMessage(
+                    jid,
+                    sid,
+                    iv,
+                    message,
+                    isPrekeyMessage,
+                    allow_untrusted=allow_untrusted,
+                )
+                body = None
+            else:
+                body = self._omemo.decryptMessage(
+                    jid,
+                    sid,
+                    iv,
+                    message,
+                    isPrekeyMessage,
+                    payload,
+                    allow_untrusted=allow_untrusted,
+                )
         except (omemo.exceptions.NoSessionException,):
             # This might happen when the sender is sending using a session
             # that we don't know about (deleted session storage, etc.). In