Fix possible panic when repository is empty (#20509)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/view.go

@@ -896,11 +896,15 @@ func renderCode(ctx *context.Context) {
 	ctx.Data["PageIsViewCode"] = true
 
 	if ctx.Repo.Repository.IsEmpty {
-		reallyEmpty, err := ctx.Repo.GitRepo.IsEmpty()
+		reallyEmpty := true
+		var err error
+		if ctx.Repo.GitRepo != nil {
+			reallyEmpty, err = ctx.Repo.GitRepo.IsEmpty()
 			if err != nil {
 				ctx.ServerError("GitRepo.IsEmpty", err)
 				return
 			}
+		}
 		if reallyEmpty {
 			ctx.HTML(http.StatusOK, tplRepoEMPTY)
 			return