Merge branch '473156-prompt-migration' into 'master'

Add feature flag to send a nil prompt to the AI gateway.

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/160050



Merged-by: Igor Drozdov <idrozdov@gitlab.com>
Approved-by: Igor Drozdov <idrozdov@gitlab.com>
Approved-by: Eduardo Bonet <ebonet@gitlab.com>
Reviewed-by: Mohamed Hamda <mhamda@gitlab.com>
Co-authored-by: Mohamed Hamda <mhamda@gitlab.com>