Fix DiffStatsType encoding error

What does this MR do and why?

Fixes Related to #375591 (DiffStatsType encoding error)

How to set up and validate locally

1. Create an MR containing a new file called romualdatchadé.yml
2. Query the MR in GraphQL:

   query {
     mergeRequest(id: "gid://gitlab/MergeRequest/123") {
       diffStats {
         path
       }
     }
   }