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

* Fix cache_collection crashing when given an empty collection

* Add tests

app/controllers/concerns/cache_concern.rb

@@ -31,7 +31,9 @@ module CacheConcern
   def cache_collection(raw, klass)
     return raw unless klass.respond_to?(:with_includes)
 
-    raw                    = raw.cache_ids.to_a if raw.is_a?(ActiveRecord::Relation)
+    raw = raw.cache_ids.to_a if raw.is_a?(ActiveRecord::Relation)
+    return [] if raw.empty?
+
     cached_keys_with_value = Rails.cache.read_multi(*raw).transform_keys(&:id)
     uncached_ids           = raw.map(&:id) - cached_keys_with_value.keys
 

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 = []