Prevent attempting to precache empty URLs which causes SW install failure

[westonruter]

Fixes #270.

At the moment when attempting to precache an empty URL such as via:

add_action( 'wp_front_service_worker', function ( WP_Service_Worker_Scripts $service_worker ) {
	$service_worker->precaching_routes()->register( '' );
} );

The result is the service worker failing to install at all.

For this reason, this PR emits a _doing_it_wrong if the $url is empty or cannot be parsed. This allows the service worker to still be installed, while the incorrect usage will appear in the error log.

Aside: It may be better to also emit the incorrect usage via console.warn() so that the issue is more easily seen.