ox: dont print empty body message

Fix https://github.com/profanity-im/profanity/issues/1693

```
@StefanKropp @DebXWoody please check `_handle_ox_chat()` I don't
understand what you are doing there.
1)
First plain is assigned `message->plain =
p_ox_gpg_decrypt(xmpp_stanza_get_text(ox));`
and then in the same if block you overwrite this with `message->plain =
xmpp_stanza_get_text(b);` without freeing the old value as far as I can
see.

2)
Sometimes even doing `message->plain = "OX error: No payload found";`.
Shouldn't there be a `strdup()`? I think later on we try to free the
whole message struct. So we can't mix this static things.
```

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