fix(protocols): fix header serde, handle unset union payloads

[kuhe]

this is a set of changes revealed by re-enabling the skipped protocol tests in aws/aws-sdk-js-v3#6512

[kuhe]

old:

  const data: Record<string, any> | undefined = __expectUnion(await parseBody(output.body, context));
  contents.nested = de_UnionPayload(data, context);

(does not handle empty output correctly)

new:

  const data: Record<string, any> | undefined = await parseBody(output.body, context);
  if (Object.keys(data ?? {}).length) {
    contents.nested = __expectUnion(de_UnionPayload(data, context));
  }

[kuhe]

This function filters the input/serialization side of list/set shape headers. A simple .join(",") is not enough, since if the individual elements contain either the delimiter , or ", they must be quoted. The internal quotes must also be escaped.

[kuhe]

This takes over .split(",") on the deserialization side. Splitting by commas is not enough, because commas within quotes do not count as delimiters, and once split, the items wrapped by double quotes should have those quotes removed.

Behavior specified here: https://github.com/smithy-lang/smithy/blob/main/smithy-aws-protocol-tests/model/restJson1/http-headers.smithy#L229-L232

{
        id: "RestJsonInputAndOutputWithQuotedStringHeaders",
        documentation: "Tests responses with string list header bindings that require quoting",
        protocol: restJson1,
        code: 200,
        headers: {
            "X-StringList": "\"b,c\", \"\\\"def\\\"\", a"
        },
        params: {
            headerStringList: ["b,c", "\"def\"", "a"]
        }
}

[kuhe]

I propose that we ignore comma delimiter spacing rather than changing the upstream Smithy test assertion or our and other language implementations.