Code Modernization: Check the return type of parse_url() in url_to_postid().

As per the PHP manual:
> If the `component` parameter is omitted, an associative array is returned.
> If the `component` parameter is specified, `parse_url()` returns a string (or an int, in the case of `PHP_URL_PORT`) instead of an array. If the requested component doesn't exist within the given URL, `null` will be returned.

Reference: [https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues PHP Manual: parse_url(): Return Values]

In this case, `parse_url()` is called with `PHP_URL_HOST` as `$component`, which returns `null` if the URL only has a path. The return value of `parse_url()` was then passed to `str_replace()`, leading to a notice on PHP 8.1:
{{{
str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
}}}

Adding validation for the return type value of `parse_url()` prevents that.

This commit addresses a few errors in the test suite along the lines of:
{{{
5) Tests_Rewrite::test_url_to_postid_home_has_path
str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated

/var/www/src/wp-includes/rewrite.php:503
/var/www/tests/phpunit/tests/rewrite.php:271
/var/www/vendor/bin/phpunit:123
}}}

Includes adding a dedicated unit test for a URL that only has a path.

Follow-up to [41786], [51606], [51622], [51626], [51629], [51630], [52799].

Props jrf, aristath, poena, justinahinon, SergeyBiryukov.
See #55656.

git-svn-id: https://develop.svn.wordpress.org/trunk@54364 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/rewrite.php

@@ -500,8 +500,21 @@ function url_to_postid( $url ) {
 	 */
 	$url = apply_filters( 'url_to_postid', $url );
 
-	$url_host      = str_replace( 'www.', '', parse_url( $url, PHP_URL_HOST ) );
-	$home_url_host = str_replace( 'www.', '', parse_url( home_url(), PHP_URL_HOST ) );
+	$url_host = parse_url( $url, PHP_URL_HOST );
+
+	if ( is_string( $url_host ) ) {
+		$url_host = str_replace( 'www.', '', $url_host );
+	} else {
+		$url_host = '';
+	}
+
+	$home_url_host = parse_url( home_url(), PHP_URL_HOST );
+
+	if ( is_string( $home_url_host ) ) {
+		$home_url_host = str_replace( 'www.', '', $home_url_host );
+	} else {
+		$home_url_host = '';
+	}
 
 	// Bail early if the URL does not belong to this site.
 	if ( $url_host && $url_host !== $home_url_host ) {

tests/phpunit/tests/rewrite.php

@@ -255,8 +255,17 @@ public function test_url_to_postid_hierarchical_with_matching_leaves() {
 		$this->assertSame( $grandchild_id_2, url_to_postid( get_permalink( $grandchild_id_2 ) ) );
 	}
 
-	public function test_url_to_postid_home_has_path() {
+	/**
+	 * @covers ::url_to_postid
+	 */
+	public function test_url_to_postid_url_has_only_path() {
+		$this->assertSame( 0, url_to_postid( '/example/' ) );
+	}
 
+	/**
+	 * @covers ::url_to_postid
+	 */
+	public function test_url_to_postid_home_has_only_path() {
 		update_option( 'home', home_url( '/example/' ) );
 
 		$id = self::factory()->post->create(