NoMethodError: undefined method `downcase' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2738889/?referrer=gitlab_plugin

NoMethodError: undefined method `downcase' for nil:NilClass
  commit.rb:293:in `lazy_author'
    BatchLoader.for(author_email.downcase).batch do |emails, loader|
  commit_collection.rb:23:in `each'
    commits.each(&block)
  commit_collection.rb:23:in `each'
    commits.each(&block)
  renders_commits.rb:25:in `prepare_commits_for_rendering'
    commits.each(&:lazy_author) # preload commits' authors
  renders_commits.rb:20:in `set_commits_for_rendering'
    prepare_commits_for_rendering(limited)
...
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