Zend\Cache\Pattern\CallbackCache doesn't work with NULL

library/Zend/Cache/Pattern/CallbackCache.php

@@ -48,7 +48,7 @@ public function call($callback, array $args = array())
         $key     = $this->generateCallbackKey($callback, $args);
         $result  = $storage->getItem($key, $success);
         if ($success) {
-            if (!isset($result[0])) {
+            if (!array_key_exists(0, $result)) {
                 throw new Exception\RuntimeException("Invalid cached data for key '{$key}'");
             }
 

tests/ZendTest/Cache/Pattern/CallbackCacheTest.php

@@ -35,6 +35,14 @@ public static function emptyMethod() {}
 
 }
 
+class FailableCallback
+{
+    public function __invoke()
+    {
+        throw new \Exception('This callback should either fail or never be invoked');
+    }
+}
+
 /**
  * Test function
  * @see ZendTest\Cache\Pattern\Foo::bar
@@ -165,4 +173,17 @@ protected function _testCall($callback, array $args)
             $this->assertEquals('', $data);
         }
     }
+
+    /**
+     * @group 4629
+     * @return void
+     */
+    public function testCallCanReturnCachedNullValues()
+    {
+        $callback = new FailableCallback();
+        $key      = $this->_pattern->generateKey($callback, array());
+        $this->_storage->setItem($key, array(null));
+        $value    = $this->_pattern->call($callback);
+        $this->assertNull($value);
+    }
 }