[Bug] Orphaned Namespace Links

Docfx creates a link for each part of a namespace, even if the overview page for a namespace does not exist, because it contains no members.

**To Reproduce**

1. Create a .NET project with a class Dummy in Namespace X.Y.Z.
2. Building a website with Docfx creates a page for the class Dummy and mentions the namespace X.Y.Z correctly.
3. Clicking on X or Y results in an HTTP 404 Not Found.

**Expected behavior**
Namespaces without members should not be used as link targets.

**Context:**
- OS: Windows, Ubuntu
- Docfx version: 2.78.5
- .NET 10