Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/lib/Drupal/Component/Plugin/Context/Context.php

@@ -67,6 +67,13 @@ class Context implements ContextInterface {
     return $this->contextValue;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function hasContextValue() {
+    return (bool) $this->contextValue || (bool) $this->getContextDefinition()->getDefaultValue();
+  }
+
   /**
    * {@inheritdoc}
    */

core/lib/Drupal/Component/Plugin/Context/ContextInterface.php

@@ -30,6 +30,14 @@ interface ContextInterface {
    */
   public function getContextValue();
 
+  /**
+   * Returns whether the context has a value.
+   *
+   * @return bool
+   *   TRUE if the context has a value, FALSE otherwise.
+   */
+  public function hasContextValue();
+
   /**
    * Sets the definition that the context must conform to.
    *

core/lib/Drupal/Component/Plugin/ContextAwarePluginBase.php

@@ -22,7 +22,7 @@ abstract class ContextAwarePluginBase extends PluginBase implements ContextAware
    *
    * @var \Drupal\Component\Plugin\Context\ContextInterface[]
    */
-  protected $context;
+  protected $context = [];
 
   /**
    * Overrides \Drupal\Component\Plugin\PluginBase::__construct().

core/lib/Drupal/Component/Plugin/composer.json

@@ -5,7 +5,7 @@
     "homepage": "https://www.drupal.org/project/drupal",
     "license": "GPL-2.0+",
     "require": {
-        "php": ">=5.4.2"
+        "php": ">=5.5.9"
     },
     "autoload": {
         "psr-0": {