Fix error when trying to access nil commit author (via GraphQL)

What does this MR do and why?

Fixes a bug caused by commits with nil emails

Related to #337243 (closed)

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Before	After

How to set up and validate locally

1. Push a commit with a nil e-mail: git commit --author "Snail Mail <>"
2. Run the following GraphQL query:

   query {
     mergeRequest(id: "gid://gitlab/MergeRequest/xxx") {
       commits {
         nodes {
           committerEmail
           author {
             id
             username
           }
         }
       }
     }
   }