Merge branch 'kassio/work-items-rolledup-dates-widget-nullable-fixed-date-graphql' into 'master'

WorkItems rolledup dates: make fixed dates nullable on graphql

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144451



Merged-by: Mehmet Emin INAC <minac@gitlab.com>
Approved-by: Eugenia Grieff <egrieff@gitlab.com>
Approved-by: Mehmet Emin INAC <minac@gitlab.com>
Reviewed-by: Kassio Borges <kborges@gitlab.com>
Co-authored-by: Kassio Borges <kassioborgesm@gmail.com>