[GraphQL] API mergeUser is null if autoMergeEnabled is false

Summary

On graphQL response, the mergeUser is null if autoMergeEnable is false systematically.

Steps to reproduce

  group(fullPath: "xxx") {
    mergeRequests(includeSubgroups: true,
                  mergedAfter: "2021-01-01", 
                  mergedBefore: "2021-03-01",
    authorUsername: "xxx") {
      count
      edges {
        cursor
        node {
          mergeUser {
            username
          }
          autoMergeEnabled
        }
      }
    }
  }
}

What is the current bug behavior?

mergeUser is null if autoMergeEnabled is false.

What is the expected correct behavior?

mergeUser should be reflect the user which has merged the MR.

Relevant logs and/or screenshots

Results of GitLab environment info

GitLab Community Edition 13.8.1