Add a test for explicit payload member and empty body responses

[sugmanue]

Background

Add a new protocol tests for the JSON REST protocol. When we have a response object with a member that's bound to the HTTP payload, such as

@output
structure HttpPayloadWithStructureInputOutput {
    @httpPayload
    nested: NestedPayload,
}

And the response body is empty, then the member MUST be set to null.

Regression for this case was reported to the Java SDK V2 and fixed on #6111.

Testing

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