Asset/Thumbnail: replace 'NULL' placeholder strings with actual null values

models/Thumbnail.php

@@ -335,7 +335,7 @@ class Thumbnail
 		if ($success)
 		{
 			$thumb_selector = $this->width . 'x' . $this->height . $this->filename_suffix;
-			$this->thumbnails[$thumb_selector] = $filename !== 'NULL' ? $filename : null;
+			$this->thumbnails[$thumb_selector] = $filename ?? null;
 
 			// For consistency, write new thumbnail filename to parent Image object.
 			// TODO: there could still be an inconsistency if multiple objects exists for the same image asset.
@@ -349,7 +349,7 @@ class Thumbnail
 
 	private function markAsQueued()
 	{
-		$this->updateDb('NULL');
+		$this->updateDb(null);
 	}
 
 	private function markAsGenerated($filename)