getExternalUrl() returns a protocol-relative URL for image styles when the public directory path is empty

Problem/Motivation

For image style derivatives,
FlysystemStreamWrapper::getExternalUrl() routes the URL back
through Drupal:

$dir_path = $this->getPublicDirectoryPath();
if ($dir_path !== NULL) {
  $base = rtrim($this->getDrupalBaseUrl(), '/');
  return $base . '/' . $dir_path . '/' . $target;
}

The NULL check is not enough: getPublicDirectoryPath() returns
'' (not NULL) when the public scheme's
public_url_base has no path component — a bare CDN or bucket
domain. The result contains a double slash:
https://example.com//styles/large/public/foo.jpg.

Once file_url_transform_relative() strips the base off, the
markup contains //styles/large/public/foo.jpg, which browsers
parse as protocol-relative: the host becomes styles and the image
never loads.

This affects both branches that build a Drupal-routed derivative URL (the
CloudFront branch and the generic non-local adapter branch).

Steps to reproduce

1. Configure a Flysystem scheme whose public_url_base is a bare
   domain (no path), used for public://.
2. Render any image with an image style.
3. Inspect the markup: src="//styles/...", image 404s / resolves
   against the wrong host.

Proposed resolution

Only insert the directory path segment when it is non-empty:

return $dir_path !== ''
  ? $base . '/' . $dir_path . '/' . $target
  : $base . '/' . $target;

Remaining tasks

• Review.
• Unit coverage in FlysystemStreamWrapperTest with
  getPublicDirectoryPath() returning ''.

User interface changes

None.

API changes

None.

Data model changes

None.