add check if empty reply

src/babillard.ml

@@ -397,6 +397,8 @@ let build_reply ~comment ~image_info ~tag_list ?parent_id user_id =
   else if List.length tag_list > 30 then Error "too much tags"
   else if List.exists (fun tag -> String.length tag > 100) tag_list then
     Error "tag too long"
+  else if Option.is_none image_info && String.length (String.trim comment) = 0
+  then Error "Your post must contain an image or a comment"
   else
     let tag_list =
       List.map String.lowercase_ascii