Handle body is None case when decrypting

Heartbeats don't have a payload and body will be None here.

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

poezio_omemo/__init__.py

@@ -139,6 +139,8 @@ class Plugin(E2EEPlugin):
                 # Always decrypt. Let us handle how we then warn the user.
                 allow_untrusted=True,
             )
+            # Heartbeats will return a None body.
+            if body is not None:
                 body = body.decode('utf-8')
         except (MissingOwnKey,):
             # The message is missing our own key, it was not encrypted for