use nullable instead of option for external api in atd

Author: yasunariw

lib/config.atd

@@ -34,9 +34,9 @@ type config = {
    shouldn't be checked into version control.  *)
 type secrets = {
   (* GitHub personal access token, if repo access requires it *)
-  ?gh_token : string option;
+  ?gh_token : string nullable;
   (* GitHub webhook token to secure the webhook *)
-  ?gh_hook_token : string option;
+  ?gh_hook_token : string nullable;
   (* Slack bot token obtained via OAuth, enabling message posting to the workspace *)
-  ?slack_access_token : string option;
+  ?slack_access_token : string nullable;
 }

lib/slack.atd

@@ -62,6 +62,6 @@ type post_message_req = {
 
 type post_message_res = {
   ok: bool;
-  ?channel: string option;
-  ?error: string option;
+  ?channel: string nullable;
+  ?error: string nullable;
 }