[BUG] Fix panic on too high page number

- Fixes a panic where the file history router would panic if the page
number was set to a page where no commits would be returned. It now
returns a 404 in such case.
- Regresion of a5b1c1b0b3
- Panic log provided by @algernon.
- Minimal integration test added.

Co-authored-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit 6a49e3f468)

routers/web/repo/commit.go

@@ -242,6 +242,12 @@ func FileHistory(ctx *context.Context) {
 		ctx.ServerError("CommitsByFileAndRange", err)
 		return
 	}
+
+	if len(commits) == 0 {
+		ctx.NotFound("CommitsByFileAndRange", nil)
+		return
+	}
+
 	oldestCommit := commits[len(commits)-1]
 
 	renamedFiles, err := git.GetCommitFileRenames(ctx, ctx.Repo.GitRepo.Path, oldestCommit.ID.String())