Make sure nil IDs to not get cast to empty string in REST API (#5068)

app/serializers/rest/application_serializer.rb

@@ -9,7 +9,7 @@ class REST::ApplicationSerializer < ActiveModel::Serializer
   end
 
   def client_id
-    object.uid.to_s
+    object.uid
   end
 
   def client_secret

app/serializers/rest/status_serializer.rb

@@ -24,11 +24,11 @@ class REST::StatusSerializer < ActiveModel::Serializer
   end
 
   def in_reply_to_id
-    object.in_reply_to_id.to_s
+    object.in_reply_to_id&.to_s
   end
 
   def in_reply_to_account_id
-    object.in_reply_to_account_id.to_s
+    object.in_reply_to_account_id&.to_s
   end
 
   def current_user?