Check delay field existence

Now there is no way to check if we have &lt;delay&gt; stanza inside message.

`msg['delay']` is always object, `bool(msg['delay'])` always `True`.
`msg['delay'].get_from()` is object and True.
`msg['delay'].get_text()` depends on server.
`msg['delay'].get_stamp()` fails on non-delayed messages with `ValueError`.

I'm returning None on non-delayed messages.

And import fix))