Menus: In Walker_Nav_Menu, Walker_Category, and Walker_Page, properly output link attributes having a legitimate "empty" value, for example an HTML data attribute with a value of zero (0).

Props nevma, AkSDvP, greenshady, SergeyBiryukov.
Fixes #47720.

git-svn-id: https://develop.svn.wordpress.org/trunk@46413 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/post/walkerPage.php

@@ -0,0 +1,87 @@
+<?php
+/**
+ * @group post
+ * @group walker
+ */
+class Tests_Post_Walker_Page extends WP_UnitTestCase {
+
+	/**
+	 * @var \Walker_Page The instance of the walker.
+	 */
+	public $walker;
+
+	/**
+	 * Setup.
+	 */
+	public function setUp() {
+		parent::setUp();
+
+		/** Walker_Page class */
+		require_once ABSPATH . 'wp-includes/class-walker-page.php';
+		$this->walker = new Walker_Page();
+	}
+
+	/**
+	 * @ticket 47720
+	 *
+	 * @dataProvider data_start_el_with_empty_attributes
+	 */
+	public function test_start_el_with_empty_attributes( $value, $expected ) {
+		$output = '';
+		$page   = $this->factory->post->create_and_get( array( 'post_type' => 'page' ) );
+		$link   = get_permalink( $page );
+
+		add_filter(
+			'page_menu_link_attributes',
+			function( $atts ) use ( $value ) {
+				$atts['data-test'] = $value;
+				return $atts;
+			}
+		);
+
+		$this->walker->start_el( $output, $page, 0 );
+
+		if ( '' !== $expected ) {
+			$expected = sprintf( ' data-test="%s"', $expected );
+		}
+
+		$this->assertSame( "<li class=\"page_item page-item-{$page->ID}\"><a href=\"{$link}\"{$expected}>{$page->post_title}</a>", $output );
+	}
+
+	public function data_start_el_with_empty_attributes() {
+		return array(
+			array(
+				'',
+				'',
+			),
+			array(
+				0,
+				'0',
+			),
+			array(
+				0.0,
+				'0',
+			),
+			array(
+				'0',
+				'0',
+			),
+			array(
+				null,
+				'',
+			),
+			array(
+				false,
+				'',
+			),
+			array(
+				true,
+				'1',
+			),
+			array(
+				array(),
+				'',
+			),
+		);
+	}
+}