Merge request commits are missing the commit author and/or committer fields, resulting in an HTTP 500 when viewing merge requests

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2918913/?referrer=gitlab_plugin

NoMethodError: undefined method `name' for nil:NilClass
  app/models/merge_request_diff_commit.rb:117:in `author_name'
    commit_author.name
  app/models/concerns/cached_commit.rb:8:in `public_send'
    hash[key] = public_send(key) # rubocop:disable GitlabSecurity/PublicSend
  app/models/concerns/cached_commit.rb:8:in `block in to_hash'
    hash[key] = public_send(key) # rubocop:disable GitlabSecurity/PublicSend
  app/models/concerns/cached_commit.rb:7:in `each'
    Gitlab::Git::Commit::SERIALIZE_KEYS.each_with_object({}) do |key, hash|
  app/models/concerns/cached_commit.rb:7:in `each_with_object'
    Gitlab::Git::Commit::SERIALIZE_KEYS.each_with_object({}) do |key, hash|
...
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