Fix: Handle empty union type

[psyclaudeZ]

Before this PR

When an empty union is defined, conjure-python will incorrectly generate code with syntax error like:

        if type_of_union is None:
            if  != 1:
                raise ValueError('a union must contain a single member')

After this PR

For backward compat, we have to handle the scenario post-hoc, i.e. after codegen. The wire spec doesn't explicitly state whether an empty union is allowed, but here I'm just throwing an exception during runtime anyway (it won't break existing generated binding per se - since they should be already broken by the syntax error) since relaxation is easier than tightening up if there's a change of plan.

==COMMIT_MSG==
Handle empty union type
==COMMIT_MSG==

Possible downsides?

[stale]

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[psyclaudeZ]

Does this make sense @BenjaminLowry ? (Consider hiding whitespace to unclutter the view)

[BenjaminLowry]

Yeah this makes sense to me

[autorelease3]

Released 4.22.0