Fix NoMethodError in edge case of emoji cache handling (#34749)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/lib/entity_cache.rb

@@ -26,7 +26,9 @@ class EntityCache
       uncached_ids << shortcode unless cached.key?(to_key(:emoji, shortcode, domain))
     end
 
-    unless uncached_ids.empty?
+    if uncached_ids.empty?
+      uncached = {}
+    else
       uncached = CustomEmoji.enabled.where(shortcode: shortcodes, domain: domain).index_by(&:shortcode)
       uncached.each_value { |item| Rails.cache.write(to_key(:emoji, item.shortcode, domain), item, expires_in: MAX_EXPIRATION) }
     end