Added a little protection to migration query

Just to be sure the query is filtered as expected to only affect
shelf-based images.

database/migrations/2022_09_02_082910_fix_shelf_cover_image_types.php

@@ -22,11 +22,13 @@ class FixShelfCoverImageTypes extends Migration
             ->pluck('image_id')
             ->values()->all();
 
+        if (count($shelfImageIds) > 0) {
             DB::table('images')
                 ->where('type', '=', 'cover_book')
                 ->whereIn('id', $shelfImageIds)
                 ->update(['type' => 'cover_bookshelf']);
         }
+    }
 
     /**
      * Reverse the migrations.