BUG Safely handle empty injector factory responses

Fixes issue with ImageBackendFactory returning null and breaking injector

src/Core/Injector/Injector.php

@@ -584,6 +584,11 @@ class Injector implements ContainerInterface
         $factory = isset($spec['factory']) ? $this->get($spec['factory']) : $this->getObjectCreator();
         $object = $factory->create($class, $constructorParams);
 
+        // Handle empty factory responses
+        if (!$object) {
+            return null;
+        }
+
         // figure out if we have a specific id set or not. In some cases, we might be instantiating objects
         // that we don't manage directly; we don't want to store these in the service cache below
         if (!$id) {

tests/php/Core/Injector/InjectorTest.php

@@ -14,6 +14,7 @@ use SilverStripe\Core\Tests\Injector\InjectorTest\CircularOne;
 use SilverStripe\Core\Tests\Injector\InjectorTest\CircularTwo;
 use SilverStripe\Core\Tests\Injector\InjectorTest\ConstructableObject;
 use SilverStripe\Core\Tests\Injector\InjectorTest\DummyRequirements;
+use SilverStripe\Core\Tests\Injector\InjectorTest\EmptyFactory;
 use SilverStripe\Core\Tests\Injector\InjectorTest\MyChildClass;
 use SilverStripe\Core\Tests\Injector\InjectorTest\MyParentClass;
 use SilverStripe\Core\Tests\Injector\InjectorTest\NeedsBothCirculars;
@@ -102,6 +103,21 @@ class InjectorTest extends SapphireTest
         );
     }
 
+    public function testEmptyFactory()
+    {
+        $this->expectException(InjectorNotFoundException::class);
+        $injector = new Injector();
+        $services = array(
+            'SomeClass' => array(
+                'class' => AnotherService::class,
+                'factory' => EmptyFactory::class,
+            )
+        );
+
+        $injector->load($services);
+        $injector->create('SomeClass');
+    }
+
     public function testConfiguredInjector()
     {
         $injector = new Injector();

tests/php/Core/Injector/InjectorTest/EmptyFactory.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace SilverStripe\Core\Tests\Injector\InjectorTest;
+
+use SilverStripe\Core\Injector\Factory;
+
+class EmptyFactory implements Factory
+{
+    public function create($service, array $params = array())
+    {
+        return null;
+    }
+}