Tester script for https://core.trac.wordpress.org/ticket/59832

test.php

<?php
/**
 * Install this file in wp-content/mu-plugins/ directory.
 */

add_action( 'init', function() {
	echo '<p>plugin_dir_url( __FILE__ ) : </p>';
	var_dump( plugin_dir_url( __FILE__ ) );
	echo "<hr>";

	echo '<p>plugin_dir_url( \'\' ) : </p>';
	var_dump( plugin_dir_url( '' ) );
	echo "<hr>";

	echo '<p>plugins_url( \'\', \'\' ) : </p>';
	var_dump( plugins_url( '', '' ) );
	echo "<hr>";

	echo '<p>wp_normalize_path( \'\' ) :</p>';
	var_dump( wp_normalize_path( '' ) );
	echo "<hr>";

	echo '<p>plugin_dir_url( null ) : </p>';
	var_dump( plugin_dir_url( null ) );
	echo "<hr>";

	echo '<p>wp_normalize_path( null ) :</p>';
	var_dump( wp_normalize_path( null ) );
	echo "<hr>";

	exit;
} );