Show error state in performance bar for requests that just return null

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If the performance bar tries to get data for a request ID with no data, the response from the server is a 200 with the body null. The frontend should handle this and show something sensible. At the moment it shows:

image

A request can return null for a bunch of reasons (non-exhaustive):

  1. It's more than 30 minutes since the request, and so its TTL has passed.
  2. The user who made the request can't see the performance bar (https://gitlab.com/gitlab-org/gitlab/blob/master/lib%2Fgitlab%2Fperformance_bar%2Fredis_adapter_when_peek_enabled.rb).
  3. The request itself isn't tracked by the performance bar (#33909 (closed)).
Edited by 🤖 GitLab Bot 🤖