Fixes an 500 error/panic if using the changed PR files API with pages
that should return empty lists because there are no items anymore.
`start-end` is then < 0 which ends in panic.
Backport https://github.com/go-gitea/gitea/pull/21602
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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: delvh <dev.lh@web.de>
qwerty287