Ignore empty response in ActivityPub::FetchRemoteStatusService (#4661)

* Ignore empty response in ActivityPub::FetchRemoteStatusService

This fixes `NoMethodError: undefined method `[]' for nil:NilClass` error.

* Check json.nil? in JsonLdHelper#supported_context?

app/helpers/jsonld_helper.rb

@@ -14,7 +14,7 @@ module JsonLdHelper
   end
 
   def supported_context?(json)
-    equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
+    !json.nil? && equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
   end
 
   def fetch_resource(uri)