Fix PATCH /repos/{owner}/{repo} panic (#14637)

Fix PATCH /repos/{owner}/{repo} panic (#14637)

* Fix a runtime error when modifying a repository through API call

Using the `PATCH /repos/{owner}/{repo}` endpoint and attempting to
modify `default_branch` on an empty repository will cause a
panic. This commit adds a check for a nil pointer before attempting
to dereference it.

* Apply suggestions from code review

* Apply suggestions from code review

* Ensure that the git repository is loaded

If you change the default branch for a repository you must change it in
git too. Therefore you must open the repository before changing the
default branch.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Allow empty repos to have their default branches changed

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Anton Khimich <anton.khimicha@mail.utoronto.ca>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Andrew Thornton <art27@cantab.net>