Servers must reject {} when deserializing a union

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.

smithy-aws-protocol-tests/model/restJson1/malformedRequests/malformed-union.smithy

@@ -78,6 +78,28 @@ apply MalformedUnion @httpMalformedRequestTests([
             }
         }
     },
+    {
+        id: "RestJsonMalformedUnionEmptyObjectNoFieldsSet",
+        documentation: """
+            When the union is an empty object, it has no fields set, so the
+            response should be a 400 SerializationException.""",
+        protocol: restJson1,
+        request: {
+            method: "POST",
+            uri: "/MalformedUnion",
+            body: """
+                { "union" : {  } }""",
+            headers: {
+                "content-type": "application/json"
+            }
+        },
+        response: {
+            code: 400,
+            headers: {
+                "x-amzn-errortype": "SerializationException"
+            }
+        }
+    },
     {
         id: "RestJsonMalformedUnionValueIsArray",
         documentation: """