Empty repositories crash github-backup #489

What happened?

Steps to reproduce:

github-backup -t github_pat_123 pski

Result:

2026-02-18T09:38:25.724: Backing up user pski to /tmp
2026-02-18T09:38:25.725: Requesting https://api.github.com/user
2026-02-18T09:38:25.966: Retrieving repositories
2026-02-18T09:38:25.966: Requesting https://api.github.com/users/pski/repos?per_page=100
2026-02-18T09:38:26.315: Filtering repositories
2026-02-18T09:38:26.315: Backing up repositories
2026-02-18T09:38:26.315: '>' not supported between instances of 'NoneType' and 'str'

The repository causing this bug is pski/Testproject which is empty.

Note that this bug will not be triggered if you directly backup the repository via github-backup -t github_pat_123 -R Testproject pski.

In line 1760 github-backup retrieves the lists of repositories for a user. In line 1777 github-backup checks the following:

elif "pushed_at" in repository and repository["pushed_at"] > last_update:

For empty repositories repository["pushed_at"] is present however it is None, so the comparison causes an Exception.

I'm sorry that I don't provide a Pull Request, I don't have the means to do that right now.