Fixed NoMethodError in UnfollowService (#1918)

app/services/unfollow_service.rb

@@ -6,6 +6,7 @@ class UnfollowService < BaseService
   # @param [Account] target_account Which to unfollow
   def call(source_account, target_account)
     follow = source_account.unfollow!(target_account)
+    return unless follow
     NotificationWorker.perform_async(build_xml(follow), source_account.id, target_account.id) unless target_account.local?
     UnmergeWorker.perform_async(target_account.id, source_account.id)
   end