Make validate support optional fields

[mkmik]

No description provided.

[dcodeIO]

Optional fields should already be checked here when present.

Are you experiencing any issues? Maybe null should be checked there, too.

[mkmik]

yes I added this check because that generated function was entered with m == null when an optional field was omitted. or at least the stack trace said that null has no such property. afk atm, I can attach a failing case when I'm back

[dcodeIO]

I assume that the null value there is representing an optional message. I added a commit above, give it a try when you're back.

[mkmik]

I confirm that master no longer exhibits the issue, abandoning this PR. Thank you!