Fix navigation render result (#1533)

The returned value should not be empty here

Author: reakaleek

src/tooling/docs-assembler/Navigation/GlobalNavigationHtmlWriter.cs

@@ -57,7 +57,13 @@ public async Task<NavigationRenderResult> RenderNavigation(IRootNavigationItem<I
 		var navigationId = ShortId.Create($"{(navigationRootSource, maxLevel).GetHashCode()}");
 
 		if (_renderedNavigationCache.TryGetValue((navigationRootSource, maxLevel), out var value))
-			return NavigationRenderResult.Empty;
+		{
+			return new NavigationRenderResult
+			{
+				Html = value,
+				Id = navigationId
+			};
+		}
 
 		if (navigationRootSource == new Uri("docs-content:///"))
 		{