Avoid dropping messages with missing ciphertext on the floor

- When messages are lacking in ciphertext (it needs to be reencoded for
  us), WA will send us a message stub (which we don't process, and need
  to).
- This then leads to us getting an empty message content, eating it
  to avoid saying something useless, storing the msgid, and then not
  sending the real message content later!
- This fixes that by checking we're actually sending something before
  storing.
  (It's only a short-term fix really though.)