Remove optionality from documents that are in unions

[jdisanti]

⚠️ Breaking Change: Unions with Documents will see the inner document type change from Option<Document> to Document.

I don't think it makes sense for Documents to be optional inside of unions. Consider the following:

struct Input {
   union: MyUnion
}
union MyUnion {
    doc: Document,
    notDoc: String,
}

Currently, this generates:

enum MyUnion {
    Doc(Option<Document>),
    NotDoc(String),
}

But when you consider the following:

// Invalid JSON
{"union":undefined}
{"union":{"doc":undefined}}

// Invalid: variant not specified
{"union":{}}

// Valid, but is Document::Null
{"union":{"doc":null}}

// Valid, but uses the other variant
{"union":{"notDoc":"something"}}

It doesn't seem possible to ever have None as the document value in the union.

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[rcoh]

this is definitely an @mtdowling question. If we decide this, it should be brought into NullableIndex

[rcoh]

LGTM, can't tell my left from my right.

[jdisanti]

Turns out the NullableIndex already states that all members in union containers should not be nullable by checking the container type and returning false if it is a union (the default case).

We were previously overriding that with the or in SymbolVisitor for reasons unknown.

I pushed another commit to the PR that simplifies handleOptionality down to just relying on the NullableIndex for this which resolves the issue.