Don't fail on decrypting heartbeats

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

slixmpp_omemo/__init__.py

@@ -650,8 +650,9 @@ 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 = None
+        if encrypted['payload']['value'] is not None:
             payload = b64dec(encrypted['payload']['value'])
 
         jid = sender.bare
@@ -674,6 +675,17 @@ 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:
+            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,