chore: differentiate restJson1 empty body behavior

This adds differentation for empty body behavior in restJson1. For
responses this protocol requires always sending an empty json object
if there is some shape defined as output but nothing bound to the
body or payload.

Author: JordonPhillips

codegen/smithy-aws-typescript-codegen/src/main/java/software/amazon/smithy/aws/typescript/codegen/RestJsonProtocolGenerator.java

@@ -89,8 +89,11 @@ public void generateSharedComponents(GenerationContext context) {
     @Override
     protected void writeDefaultHeaders(GenerationContext context, Shape operationOrError, boolean isInput) {
         super.writeDefaultHeaders(context, operationOrError, isInput);
-        if (isInput && operationOrError.isOperationShape()) {
-            AwsProtocolUtils.generateUnsignedPayloadSigV4Header(context, operationOrError.asOperationShape().get());
+        if (operationOrError.isOperationShape()) {
+            OperationShape operation = operationOrError.asOperationShape().get();
+            if (isInput) {
+                AwsProtocolUtils.generateUnsignedPayloadSigV4Header(context, operation);
+            }
         } else if (operationOrError.isStructureShape()) {
             context.getWriter().write("'x-amzn-errortype': $S,", operationOrError.getId().getName());
         }
@@ -106,7 +109,11 @@ public void serializeDocumentBody(
         // Short circuit when we have no bindings.
         TypeScriptWriter writer = context.getWriter();
         if (documentBindings.isEmpty()) {
-            writer.write("body = \"\";");
+            if (isInput) {
+                writer.write("body = \"\";");
+            } else {
+                writer.write("body = \"{}\";");
+            }
             return;
         }
 
@@ -172,6 +179,19 @@ private DocumentMemberSerVisitor getMemberSerVisitor(GenerationContext context,
         return new JsonMemberSerVisitor(context, dataSource, getDocumentTimestampFormat());
     }
 
+    @Override
+    protected boolean shouldWriteDefaultBody(GenerationContext context, Shape operationOrError, boolean isInput) {
+        if (!isInput) {
+            // Operations that have any defined output shape should always send a default body.
+            if (operationOrError.isOperationShape()) {
+                OperationShape operation = operationOrError.asOperationShape().get();
+                return operation.getOutput().isPresent();
+            }
+            return true;
+        }
+        return super.shouldWriteDefaultBody(context, operationOrError, isInput);
+    }
+
     @Override
     protected void writeErrorCodeParser(GenerationContext context) {
         TypeScriptWriter writer = context.getWriter();