Merge branch 'fix-merge-train-graphql-nre' into 'master'

Fix NRE when logged out and reading merge trains via GraphQL

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167410



Merged-by: Lee Tickett <ltickett@gitlab.com>
Approved-by: Niklas van Schrick <mc.taucher2003@gmail.com>
Approved-by: Stan Hu <stanhu@gmail.com>
Reviewed-by: Niklas van Schrick <mc.taucher2003@gmail.com>