Bugfix extra autoroute slashes

[MGatner]

Description
When controller and method are both empty and there is no explicit default route (i.e. $routes->get('/', ...) then Router will call validateRequest() with two blank entries in $segments, which in turn calls setDirectory('') twice resulting in a directory of //. This causes autoroute() to fail to locate existing controllers.

This PR has setDirectory() ignore empty requests, since $this->directory of null is already equivalent to /.

Fix for #2301

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide