fix(oauth2): Fixed Gmail API OAuth2 schema

Author: andris9

lib/schemas.js

@@ -1215,7 +1215,7 @@ const oauthCreateSchema = {
         .base64({ paddingRequired: false, urlSafe: true })
         .max(512)
         .example('AAAAAQAACnA')
-        .valid(false, null)
+        .allow(false, null)
         .description('Cloud Pub/Sub app for Gmail API webhooks'),
 
     extraScopes: Joi.any()