Navigation buttons when posts are grouped monthly allow to go before 1st post date

If the blog setting Index display: is set to Group posts monthly navigation buttons « Previous month and Next month » will be displayed.

It doesn't make sense to have a button « Previous month (or at least it should be disabled) if we reach the first blog post ever published. In a same way it doesn't make sense to display Next month » button if we reach the latest blog post published (or latest draft for a user who have edit privilege).

The issue related to keeping those links can arise if a crawler (or a link checker) try to index pages following the links on those buttons: it will end up in an infinite loop.

Relevant code is located here: https://github.com/xwiki-contrib/application-blog/blob/47f580ac2ba6595f49548a35c4f627b8a33285a8/application-blog-ui/src/main/resources/Blog/BlogCode.xml#L892

The same situation probably happen with Group posts weekly.