Issue #3401128: Ensure empty array is passed instead of NULL when no definitions are found.

Closes #3401128

src/PluginManager/InstallablePluginManager.php

@@ -382,10 +382,17 @@ abstract class InstallablePluginManager extends DefaultPluginManager implements
     static::$runtimeDefinitions[static::class] = $this->getCachedDefinitions(TRUE);
 
     // Build the runtime definitions.
-    if (!isset(static::$runtimeDefinitions[static::class])) {
+    if (empty(static::$runtimeDefinitions[static::class])) {
       // Retrieve normal definitions.
       static::$runtimeDefinitions[static::class] = parent::getDefinitions();
 
+      if (empty(static::$runtimeDefinitions[static::class])) {
+        // Return empty array if no definitions were found.
+        $this->definitions = [];
+
+        return $this->definitions;
+      }
+
       // Validate runtime definition requirements.
       /** @var \Drupal\markdown\Annotation\InstallablePlugin $definition */
       foreach (static::$runtimeDefinitions[static::class] as $definition) {