Fix cache_collection crashing when given an empty collection (#15921)

* Fix cache_collection crashing when given an empty collection

* Add tests

app/lib/entity_cache.rb

@@ -16,7 +16,9 @@ class EntityCache
   end
 
   def emoji(shortcodes, domain)
-    shortcodes   = Array(shortcodes)
+    shortcodes = Array(shortcodes)
+    return [] if shortcodes.empty?
+
     cached       = Rails.cache.read_multi(*shortcodes.map { |shortcode| to_key(:emoji, shortcode, domain) })
     uncached_ids = []