Allow/fix review (approve/reject) of empty PRs (#25690) (#25732)

Backport #25690 by @sebastian-sauer

gitea allows to create empty PRs.

Currently when you need approvals for a merge, you have to manually add
/files to the url to get to the files tab to approve / reject the PR.

This PR allows to open the files tab via the normal tab / link and then
fixes the layout of the files tab.

**Screenshots:**

Before:

![image](https://github.com/go-gitea/gitea/assets/1135157/b5082e5e-8c32-4412-993e-b854905e96d3)

After:

![image](https://github.com/go-gitea/gitea/assets/1135157/1f5e056e-396f-4dfb-8d14-e17a2f6495d9)

Co-authored-by: sebastian-sauer <[email protected]>
Co-authored-by: silverwind <[email protected]>