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

* Fix cache_collection crashing when given an empty collection

* Add tests

spec/lib/entity_cache_spec.rb

@@ -0,0 +1,19 @@
+require 'rails_helper'
+
+RSpec.describe EntityCache do
+  let(:local_account)  { Fabricate(:account, domain: nil, username: 'alice') }
+  let(:remote_account) { Fabricate(:account, domain: 'remote.test', username: 'bob', url: 'https://remote.test/') }
+
+  describe '#emoji' do
+    subject { EntityCache.instance.emoji(shortcodes, domain) }
+
+    context 'called with an empty list of shortcodes' do
+      let(:shortcodes) { [] }
+      let(:domain)     { 'example.org' }
+
+      it 'returns an empty array' do
+        is_expected.to eq []
+      end
+    end
+  end
+end