UpdateAllRemoteMirrorsWorker can fail if `project` is `nil`

Created by @stanhu.

Suspect a deleted project is causing retries.

https://sentry.gitlap.com/gitlab/gitlabcom/issues/12793/

NoMethodError: undefined method `import_url' for nil:NilClass
  app/models/remote_mirror.rb:112:in `url_availability'
    if project.import_url == url && project.mirror?
  app/models/remote_mirror.rb:83:in `mark_as_failed'
    update_fail
  lib/gitlab/metrics/instrumentation.rb:155:in `block in mark_as_failed'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
  lib/gitlab/metrics/instrumentation.rb:155:in `mark_as_failed'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
...
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