Set as optional argument

Signed-off-by: Vitor Mattos <vitor@php.rio>

lib/private/Comments/Comment.php

@@ -352,7 +352,7 @@ class Comment implements IComment {
 	/**
 	 * @inheritDoc
 	 */
-	public function setLatestChildDateTime(\DateTime $dateTime = null) {
+	public function setLatestChildDateTime(?\DateTime $dateTime = null) {
 		$this->data['latestChildDT'] = $dateTime;
 		return $this;
 	}

lib/public/Comments/IComment.php

@@ -234,7 +234,7 @@ interface IComment {
 	 * @return IComment
 	 * @since 9.0.0
 	 */
-	public function setLatestChildDateTime(\DateTime $dateTime = null);
+	public function setLatestChildDateTime(?\DateTime $dateTime = null);
 
 	/**
 	 * returns the object type the comment is attached to