When YOURLS replies with statusCode 200 but no "shorturl" field,
YourlsProxy::_extractShortUrl() returned int 0 for its ?string return
type. Under the file's declare(strict_types=1) this raises an uncaught
TypeError, aborting paste creation instead of surfacing the intended
"Error parsing proxy response" message. Fall back to null (matching
ShlinkProxy) so the existing graceful error path is taken.
Co-Authored-By: Claude <noreply@anthropic.com>