Update opinionated protocol tests to not describe optional behavior (s…

…mithy-lang#481)

* Update opinionated protocol tests to not describe optional behavior

Removes the protocol tests for HTTP Prefix Header deserialization,
deserializing to an empty map vs null value. Null vs empty map will be
SDK implementation specific distinction. If there was a way to model
`empty` as an expectation that would encompass both null and empty but
initialized map this test would be more portable.

Updates the empty input output serialization protocol test to expect
empty payload not empty JSON object as the serialized result. It doesn't
look like the REST-JSON protocol should be serializing empty JSON
object when no members are serialized.

smithy-aws-protocol-tests/model/restJson1/empty-input-output.smithy

@@ -83,7 +83,7 @@ apply EmptyInputAndEmptyOutput @httpRequestTests([
         protocol: restJson1,
         method: "POST",
         uri: "/EmptyInputAndEmptyOutput",
-        body: "{}",
+        body: "",
         bodyMediaType: "application/json"
     },
 ])
@@ -97,6 +97,14 @@ apply EmptyInputAndEmptyOutput @httpResponseTests([
         body: "",
         bodyMediaType: "application/json"
     },
+    {
+        id: "RestJsonEmptyInputAndEmptyJsonObjectOutput",
+        documentation: "Empty output serializes no payload",
+        protocol: restJson1,
+        code: 200,
+        body: "{}",
+        bodyMediaType: "application/json"
+    },
 ])
 
 structure EmptyInputAndEmptyOutputInput {}

smithy-aws-protocol-tests/model/restJson1/http-prefix-headers.smithy

@@ -76,20 +76,6 @@ apply HttpPrefixHeaders @httpResponseTests([
             }
         }
     },
-    {
-        id: "RestJsonHttpPrefixHeadersAreNotPresent",
-        documentation: "No prefix headers are serialized because the value is empty",
-        protocol: restJson1,
-        code: 200,
-        body: "",
-        headers: {
-            "X-Foo": "Foo"
-        },
-        params: {
-            foo: "Foo",
-            fooMap: {}
-        }
-    },
 ])
 
 structure HttpPrefixHeadersInputOutput {