echo_client: Handle decrypt_message's Optional-ness

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

examples/echo_client.py

@@ -150,6 +150,11 @@ class EchoBot(ClientXMPP):
         try:
             encrypted = msg['omemo_encrypted']
             body = self['xep_0384'].decrypt_message(encrypted, mfrom, allow_untrusted)
+            # decrypt_message returns Optional[str]. It is possible to get
+            # body-less OMEMO message (see KeyTransportMessages), currently
+            # used for example to send heartbeats to other devices.
+            if body is None:
+                return None
             decoded = body.decode('utf8')
             if self.is_command(decoded):
                 await self.handle_command(mto, mtype, decoded)