Skip wiki creation when GitHub project has wiki enabled

## What does this MR do?

When importing a repository from if the repository has wiki, we should not create the default wiki. Otherwise the GitHub importer will fail because the wiki repository already exist. This bug was introduced here https://gitlab.com/gitlab-org/gitlab-ce/commit/892dea67717c0efbd6a28f7639f34535ec0a8747

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

No.

## Why was this MR needed?

GitLab fails to import GitHub Wiki.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [X] Added for this feature/bug
  - [ ] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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)

## What are the relevant issue numbers?

Fixes #22225

See merge request !6665
Signed-off-by: Rémy Coutable <remy@rymai.me>