[GITEA] Fix NPE in UsernameSubRoute

- When the user is not found in `reloadparam`, early return when the
user is not found to avoid calling `IsUserVisibleToViewer` which in turn
avoids causing a NPE.
- This fixes the case that a 500 error and 404 error is shown on the
same page.
- Add integration test for non-existant user RSS.
- Regression by c6366089df

(cherry picked from commit f0e0696278)
(cherry picked from commit 75d8066908)

routers/web/user/home.go

@@ -710,12 +710,15 @@ func UsernameSubRoute(ctx *context.Context) {
 	reloadParam := func(suffix string) (success bool) {
 		ctx.SetParams("username", strings.TrimSuffix(username, suffix))
 		context_service.UserAssignmentWeb()(ctx)
+		if ctx.Written() {
+			return false
+		}
 		// check view permissions
 		if !user_model.IsUserVisibleToViewer(ctx, ctx.ContextUser, ctx.Doer) {
 			ctx.NotFound("user", fmt.Errorf(ctx.ContextUser.Name))
 			return false
 		}
-		return !ctx.Written()
+		return true
 	}
 	switch {
 	case strings.HasSuffix(username, ".png"):