diff --git a/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
index e245874..28116ec 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DerivativeDiscoveryDecorator.php
@@ -38,7 +38,7 @@ class DerivativeDiscoveryDecorator implements DiscoveryInterface {
     list($base_plugin_id, $derivative_id) = $this->decodePluginId($plugin_id);
 
     $plugin_definition = $this->decorated->getDefinition($base_plugin_id);
-    if (isset($plugin_definition)) {
+    if ($plugin_definition) {
       $derivative_fetcher = $this->getDerivativeFetcher($base_plugin_id, $plugin_definition);
       if ($derivative_fetcher) {
         $plugin_definition = $derivative_fetcher->getDerivativeDefinition($derivative_id, $plugin_definition);
diff --git a/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
index de74fb5..a749c2e 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/StaticDiscovery.php
@@ -24,7 +24,7 @@ class StaticDiscovery implements DiscoveryInterface {
    * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinition().
    */
   public function getDefinition($base_plugin_id) {
-    return isset($this->definitions[$base_plugin_id]) ? $this->definitions[$base_plugin_id] : NULL;
+    return isset($this->definitions[$base_plugin_id]) ? $this->definitions[$base_plugin_id] : array();
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Plugin/PluginManagerBase.php b/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
index 73f1b55..61a7922 100644
--- a/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
+++ b/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
@@ -47,7 +47,7 @@ abstract class PluginManagerBase implements PluginManagerInterface {
    */
   public function getDefinition($plugin_id) {
     $definition = $this->discovery->getDefinition($plugin_id);
-    if (isset($definition)) {
+    if ($definition) {
       $this->processDefinition($definition, $plugin_id);
     }
     return $definition;