fix: Handle no remote gracefully

[chme]

Fix for #24

With this change, git-changelog does not fail any more, if it cannot find a remote.
The generated changelog is not perfect. All the links have no target url.
Given that this is an edge case, maybe this is good enough?

Example output without remote:

## [1.0.0]() - 2024-03-02

<small>[Compare with 0.1.0]()</small>

### Features

- New feature for planned v1.0.0 ([3dba998]() by Christian Meffert).

## [0.1.0]() - 2024-03-02

<small>[Compare with first commit]()</small>

[pawamoy]

Hmmm, I'm tempted to say LGTM, though maybe it's better if we handle this in templates too, to avoid links that lead to nowhere 🤔 OK you know what, let go with this, we can always change things if users request it. Thanks a lot!

[pawamoy]

🚀