Servers must reject {} when deserializing a union

[david-perez]

Once we're inside of a {} JSON object, a known union variant must be
provided. Servers must not accept {} as "the union shape was not set"
when the union shape is optional (which was the behavior in smithy-rs
prior to smithy-lang/smithy-rs#3481).

This commit adds a protocol test to pin this behavior.

Testing

I verified that the added test fails in smithy-rs when checking out a commit
prior to smithy-lang/smithy-rs#3481

thread 'operation::server_union_with_empty_body_operation_test::rest_json_with_empty_json_object_for_union_variant_malformed_request' panicked at 'request should have been rejected, but we accepted it; we parsed operation input `UnionWithEmptyBodyOperationInput { member: None }`', rest_json_extras/rust-server-codegen/src/operation.rs:774:52

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[david-perez]

This applies to all JSON-based protocols. But it seems like the repo only has @httpMalformedRequestTests for restJson1.

[david-perez]

Note that clients should reject {} too when deserializing a union in a response (instead of setting an "unknown union variant"). But there's no @httpMalformedResponseTests to pin this behavior :(