Does not handle repositories with no commits

[jangerhard]

Describe the bug
The component does not handle repositories with no commits, as "ref": null under the node for the given repository.

To Reproduce
Steps to reproduce the behavior:

1. Have a newly created repository created for a given user, with no commits
2. Load component
3. See 404 errors in console, and an empty component.

Expected behavior
The component should instead render an error-message or indicate to the user that no commits were found for the given repository.

Additional context
The 404 error in the console should have been handled by issue #8.

[jangerhard]

Closed in #19