Prevent the first navbar element from being set as active unnecessarily in the modern template

[KubaZ2]

Fixes #9877

[yufeih]

Thank you @KubaZ2

[filzrev]

This PR introduce some behavior changes on existing docfx site.

Currently if there is no matching navigation element exists. first element is selected by default.
But after this PR merged. no element is activated.

Example URLs

• https://dotnet.github.io/docfx/index.html
• https://dotnet.github.io/docfx/reference/docfx-json-reference.html

When opening above URLs.
Docs navigation is activated before PR is merged.
But after this PR is merged. no navigation element is activated.

I'm not particularly concerned about the differences in behavior.
But it should be documented as BREAKING CHANGES on release note.