Merge branch '438908-design-management-update-sidebar-empty-state' into 'master'

Add empty state for discussion in sidebar

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/147642



Merged-by: Florie Guibert <fguibert@gitlab.com>
Approved-by: Nick Brandt <nbrandt@gitlab.com>
Approved-by: Lee Tickett <ltickett@gitlab.com>
Approved-by: Florie Guibert <fguibert@gitlab.com>
Reviewed-by: Lee Tickett <ltickett@gitlab.com>
Co-authored-by: Rajan Mistry <rmistry@gitlab.com>