fix: check error header (#685)

Author: Chase Coalwell

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

@@ -110,8 +110,9 @@ private DocumentMemberSerVisitor getMemberSerVisitor(GenerationContext context,
     @Override
     protected void writeErrorCodeParser(GenerationContext context) {
         TypeScriptWriter writer = context.getWriter();
-
-        writer.write("errorCode = output.headers[\"x-amzn-errortype\"].split(':')[0];");
+        writer.openBlock("if (output.headers[\"x-amzn-errortype\"]) {", "}", () -> {
+            writer.write("errorCode = output.headers[\"x-amzn-errortype\"].split(':')[0];");
+        });
     }
 
     @Override