It is possible to create draft releases prior to creating a tag. This will cause a
500 on the releases page due to compare page failing.
This PR only shows the compare button if there is a SHA1 present.
Fix#16610
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>