ResourceResponse can't serialize empty array

1. +++ b/core/modules/rest/src/RequestHandler.php
   @@ -103,26 +103,31 @@ public function handle(RouteMatchInterface $route_match, Request $request) {
   +      $data = $response->getResponseData();
   +      if (!is_scalar($data)) {
   

   What happens in the "else" case?

   AFAICT there's no "else" case. If so, then why indent all this code, why not just add this extra condition in the original if-test?

2. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,146 @@
   +//    /** @var \Drupal\rest\Plugin\Type\ResourcePluginManager $plugin_manager */
   +//    $plugin_manager = $this->container->get('plugin.manager.rest');
   

   .

3. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,146 @@
   +  public function testBaseHandler() {
   +
   +    $request = new Request();
   

   Extraneous \n.

4. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,146 @@
   +    // Assert get plugin method is called.
   

   Useless comment?

5. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,146 @@
   +    // Setup stubbed out plugin manager that will return our plugin.
   ...
   +    // Setup stubbed out plugin manager that will return our plugin.
   ...
   +    // Setup stubbed out plugin manager that will return our plugin.
   

   s/Setup stubbed out/Stub/

6. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,146 @@
   +    // Response returns null this time.
   

   Why exactly does it return NULL?

   Also, "returns null" sounds weird, why not say "$response is NULL" this time, and use strict equality?

1. +++ b/core/modules/rest/src/RequestHandler.php
   @@ -103,7 +103,8 @@ public function handle(RouteMatchInterface $route_match, Request $request) {
        // Serialize the outgoing data for the response, if available.
   -    if ($response instanceof ResourceResponse && $data = $response->getResponseData()) {
   +    if ($response instanceof ResourceResponse) {
   

   The comment no longer makes sense now. Let's omit the , if available.? Or, actually, let's remove that entire comment now, because it's no longer necessary?

2. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,135 @@
   +    // a ResourceResponse so it is passed through directory.
   

   directly?

3. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,135 @@
   +  public function testSerialization($data) {
   ...
   +    $this->assertEquals(json_encode($data), $handler_response->getContent());
   

   Now it's much clearer what this tests: it simply tests that whatever is passed in is also what comes out. That makes a lot of sense :)

4. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,135 @@
   +      ['Complex \ string $%^&@ with unicode ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΣὨ'],
   

   :)

5. +++ b/core/modules/rest/tests/src/Kernel/RequestHandlerTest.php
   @@ -0,0 +1,135 @@
   +      // TODO Not supported. https://www.drupal.org/node/2427811
   

   s/TODO/@todo/

Committed/pushed to 8.1.x, thanks!

This looks like it would probably be fine for a patch release too, so re-open if you particularly want to see it in 8.0.x

I agree there shouldn't be any harm in cherry picking this for 8.0.x. Committed f32314a and pushed to 8.0.x. Thanks!