fix: make sure metadata sent for bodyless responses

The streaming server was not returning the response metadata unless the
body had some content to write to the stream. The fix is to always
force a write with an empty string.

lambda/serverless_streaming.js

@@ -69,12 +69,16 @@ export const handler = awslambda.streamifyResponse(
 
     responseStream = awslambda.HttpResponseStream.from(responseStream, metadata)
 
+    // Call write on the stream to trigger metadata to be sent
+    // https://github.com/aws/aws-lambda-nodejs-runtime-interface-client/blob/2ce88619fd176a5823bc5f38c5484d1cbdf95717/src/HttpResponseStream.js#L22
+    responseStream.write('')
+
     if (rendered) {
       setResponse(responseStream, rendered)
     } else {
       responseStream.end()
     }
-  }
+  },
 )
 
 // Copyright (c) 2020 [these people](https://github.com/sveltejs/kit/graphs/contributors) (MIT)
@@ -116,7 +120,7 @@ export async function setResponse(res, response) {
   if (response.body.locked) {
     res.end(
       'Fatal error: Response body is locked. ' +
-        `This can happen when the response was already read (for example through 'response.json()' or 'response.text()').`
+        `This can happen when the response was already read (for example through 'response.json()' or 'response.text()').`,
     )
     return
   }