Bug - [v6 Nav] - horizontal subnav scroll button 

**Describe the problem**
A clear and concise description of the problem. Which components are affected?
Follow up to comment in PR  https://github.com/patternfly/patternfly-react/pull/9948#pullrequestreview-1818940554

Nav scroll buttons always showing.  They should be hidden when there is no scrollable content. 