Image style routes get a '//styles/...' path when the public scheme's directory path is empty

Problem/Motivation

FlysystemRouteSubscriber builds the image style route paths by
concatenating a leading slash with the public stream wrapper's directory path:

'/' . $directory_path . '/styles/{image_style}/{scheme}'

and it also rewrites core's image.style_public route the same
way.

When the public scheme is itself Flysystem-backed and its
public_url_base is a bare host with no path component (a CDN or
bucket domain such as https://cdn.example.com),
getDirectoryPath() legitimately returns '' and the
route path becomes //styles/{image_style}/{scheme}.

Two things break:

1. The compiled route no longer matches the single-slash URL the site actually
   generates, so derivative requests 404.
2. Any src/href starting with // is
   parsed by browsers as protocol-relative, so the host becomes
   styles and the request never reaches the site.

Steps to reproduce

1. Configure a Flysystem scheme with the aws_s3 (or any non-local)
   driver whose public_url_base has no path component, and use it
   to serve public://.
2. Clear caches and request an image style derivative that has not been
   generated yet.
3. 404. drush router:info (or the route dump) shows the route path
   as //styles/....

Proposed resolution

Collapse repeated leading slashes when building both route paths:

$public_route->setPath(preg_replace('#^/+#', '/', $public_route->getPath()));
// ...
preg_replace('#^/+#', '/', '/' . $directory_path . '/styles/{image_style}/{scheme}')

Remaining tasks

• Review.
• Kernel/unit coverage for the route subscriber with
  getDirectoryPath() returning ''.

User interface changes

None.

API changes

None.

Data model changes

None.