Fix invalid declaration error

Related to #3408813

src/ConfigEntityDescriptionTrait.php

@@ -10,8 +10,8 @@ trait ConfigEntityDescriptionTrait {
   /**
    * Implements EntityDescriptionInterface::getDescription().
    */
-  public function getDescription() {
-    return $this->getThirdPartySetting('lightning_core', 'description');
+  public function getDescription(): string {
+    return (string) $this->getThirdPartySetting('lightning_core', 'description');
   }
 
   /**