NoMethodError: undefined method `find' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/3309439/?referrer=gitlab_plugin

NoMethodError: undefined method `find' for nil:NilClass
  lib/gitlab/email/receiver.rb:81:in `key_from_to_header'
    mail.to.find do |address|
  lib/gitlab/email/receiver.rb:76:in `block in mail_key'
    key_from_to_header || key_from_additional_headers
  lib/gitlab/utils/strong_memoize.rb:30:in `strong_memoize'
    instance_variable_set(key, yield)
  lib/gitlab/email/receiver.rb:75:in `mail_key'
    strong_memoize(:mail_key) do
  lib/gitlab/email/receiver.rb:62:in `find_handler'
    Handler.for(mail, mail_key)
...
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