feat: store null instead of empty string for album.description (#30123)

Addresses the first column of issue #28832: album.description now stores
and returns null instead of an empty string.

Co-authored-by: Giacomo Pinato <giacomo.pinato@gmail.com>

web/src/lib/modals/AlbumEditModal.svelte

@@ -17,7 +17,7 @@
   let description = $state(album.description);
 
   const onSubmit = async () => {
-    const success = await handleUpdateAlbum(album, { albumName, description });
+    const success = await handleUpdateAlbum(album, { albumName, description: description || null });
     if (success) {
       onClose();
     }

web/src/routes/(user)/albums/[albumId=id]/[[photos=photos]]/[[assetId=id]]/AlbumDescription.svelte

@@ -20,7 +20,7 @@
       const response = await updateAlbumInfo({
         id,
         updateAlbumDto: {
-          description,
+          description: description || null,
         },
       });
       eventManager.emit('AlbumUpdate', response);