Merge branch '18757-fix' into 'master'

Fallback to group's owners/masters when a project has none for the "access requested email"

## What does this MR do?

From b31c5052:

    Fallback to group's owners/masters when a project has none
    
    A project in a group can have no explicit owners/masters,
    in that case we fallbacks to the group's owners/masters.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

Because of #18757.

## What are the relevant issue numbers?

Fixes #18757.

## Does this MR meet the acceptance criteria?

- [x] No need for CHANGELOG.
- [x] No need for documentation.
- [x] No API support added.
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4791