fix: minor fix newsletter

data/siteMetadata.js

@@ -25,7 +25,7 @@ const siteMetadata = {
   newsletter: {
     // supports mailchimp, buttondown, convertkit
     // Please add  your .env file and modify it according to your selection
-    provider: '',
+    provider: 'convertkit',
   },
   comment: {
     // Select a provider and use the environment variables associated to it