Skip serializing replaces field if None

[styris-ame]

Fixes #135

[djc]

Thanks! Preparing release in #137.

[cpu]

Are there other places we should be giving the same treatment? E.g. in #135 @styris-ame said:

Now when replaces is None, it is simply not serialized, solving the issue with ZeroSSL:
{"identifiers":[{"type":"dns","value":"zerossl-test.styris.net"}],"profile":null}

Should profile be serialized when null?

[cpu]

Should profile be serialized when null?

I did a quick pass and this seems to be the only other Optional field without the annotation in a Serializable type we send to a peer. I'll open a quick fix PR in a sec.