Allow *UrlFormat to return null to not render link

[fabb]

We'd like to keep the repo urls out of the changelog, it would be helpful if the *UrlFormat settings would interpret an empty string or null as a signal to not render a link.

So e.g. when setting compareUrlFormat: null, the output should be ### 2.2.38 (2020-10-20) instead of ### [2.2.38](https://this.is.the/default/url) (2020-10-20)