Add commit check to GET api/v4/projects

What does this MR do and why?

Part 1/2 for this issue: #349972 (closed)

This fixes the most common failure for the endpoint in Sentry, which happens when using a project with a repository and no commits. It currently returns a 500.

How to set up and validate locally

Create a project with no commits:
curl --request POST --header "PRIVATE-TOKEN: <PRIVATE TOKEN>" 'http://localhost:3000/api/v4/projects' --data "name=PROJECTNAME"

Lint:
curl --header "Content-Type: application/json" --header "PRIVATE-TOKEN: <PRIVATE TOKEN>" "http://localhost:3000/api/v4/projects/:id/ci/lint"

You should get an error that is accurate: Please provide content of .gitlab-ci.yml.

MR acceptance checklist

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