[v9.2.x] Navigation: Fix crash when Help is disabled (#59375)

Navigation: Set navtree to  an empty array instead of null (#58919)

set navtree to  an empty array instead of null

(cherry picked from commit 4aa5dea96b)

Co-authored-by: Leo <108552997+lpskdl@users.noreply.github.com>

pkg/services/navtree/navtreeimpl/navtree.go

@@ -220,6 +220,10 @@ func (s *ServiceImpl) GetNavTree(c *models.ReqContext, hasEditPerm bool, prefs *
 
 	navTree = s.addHelpLinks(navTree, c)
 
+	if len(navTree) < 1 {
+		navTree = make([]*navtree.NavLink, 0)
+	}
+
 	return navTree, nil
 }