Playlist*: use nullptr instead of NULL

src/PlaylistFile.hxx

@@ -47,7 +47,7 @@ spl_valid_name(const char *name_utf8);
 
 /**
  * Returns a list of stored_playlist_info struct pointers.  Returns
- * NULL if an error occurred.
+ * nullptr if an error occurred.
  */
 PlaylistVector
 ListPlaylistFiles(Error &error);