Issue #3616488: Collapse double-slash image style routes

Luisrequested to merge
issue/flysystem-3616488:3616488-image-style-routes into 3.0.x

When the public scheme directory path is empty (bare CDN/bucket domain), concatenating '/' . $directory_path . '/styles/...' yields '//styles/...'. Browsers treat that as protocol-relative and the compiled route fails to match single-slash requests.

Co-authored-by: Cursor cursoragent@cursor.com

Closes #3616488

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