Fix processing poll Updates (#10333)

ActivityPub::ProcessPollService was checking the JSON-LD context although
it was passed only the `Question` object embedded in the `Update` activity.

app/services/activitypub/fetch_remote_poll_service.rb

@@ -5,6 +5,7 @@ class ActivityPub::FetchRemotePollService < BaseService
 
   def call(poll, on_behalf_of = nil)
     json = fetch_resource(poll.status.uri, true, on_behalf_of)
+    return unless supported_context?(json)
     ActivityPub::ProcessPollService.new.call(poll, json)
   end
 end