Language level migration.

Author: Martin Brecht-Precht

src/Common/NodeInterface.php

@@ -20,7 +20,7 @@ interface NodeInterface
 	/**
 	 * @return int
 	 */
-	public function getIndex(): int;
+	public function getIndex(): ?int;
 
 	/**
 	 * @return string

src/Content/ContentInterface.php

@@ -53,6 +53,6 @@ public function onDetach();
 	/**
 	 * @param HandlerInterface $markdomHandler
 	 */
-	public function onHandle(HandlerInterface $markdomHandler);
+	public function onHandle(HandlerInterface $markdomHandler): void;
 
 }