PHP 8.1: WP::parse_request(): prevent "passing null to non-nullable" notice

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.

Ref: https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues

In this case, `parse_url()` is called with the `PHP_URL_PATH` as `$component`. This will return `null` in the majority of cases, as - expect for subdirectory-based sites - `home_url()` will return a URL without trailing slash, like `http://example.org`.

The return value of `parse_url()` was subsequently passed to `trim()` leading to a `trim(): Passing null to parameter #1 ($string) of type string is deprecated` deprecation notice on PHP 8.1.

Fixed by adjusting the logic flow to:
* Only pass the return value of `parse_url()` to follow-on functions if it makes sense, i.e. if it isn't `null`, nor an empty string.
* Preventing calls to `preg_replace()` and `trim()` further down in the function logic flow, when `preg_replace()`/`trim()` would have nothing to do anyhow.

Author: jrfnl

src/wp-includes/class-wp.php

@@ -170,8 +170,13 @@ public function parse_request( $extra_query_vars = '' ) {
 
 			list( $req_uri ) = explode( '?', $_SERVER['REQUEST_URI'] );
 			$self            = $_SERVER['PHP_SELF'];
-			$home_path       = trim( parse_url( home_url(), PHP_URL_PATH ), '/' );
-			$home_path_regex = sprintf( '|^%s|i', preg_quote( $home_path, '|' ) );
+
+			$home_path       = parse_url( home_url(), PHP_URL_PATH );
+			$home_path_regex = '';
+			if ( is_string( $home_path ) && '' !== $home_path ) {
+				$home_path       = trim( $home_path, '/' );
+				$home_path_regex = sprintf( '|^%s|i', preg_quote( $home_path, '|' ) );
+			}
 
 			/*
 			 * Trim path info from the end and the leading home path from the front.
@@ -180,14 +185,17 @@ public function parse_request( $extra_query_vars = '' ) {
 			 */
 			$req_uri  = str_replace( $pathinfo, '', $req_uri );
 			$req_uri  = trim( $req_uri, '/' );
-			$req_uri  = preg_replace( $home_path_regex, '', $req_uri );
-			$req_uri  = trim( $req_uri, '/' );
-			$pathinfo = trim( $pathinfo, '/' );
-			$pathinfo = preg_replace( $home_path_regex, '', $pathinfo );
 			$pathinfo = trim( $pathinfo, '/' );
 			$self     = trim( $self, '/' );
-			$self     = preg_replace( $home_path_regex, '', $self );
-			$self     = trim( $self, '/' );
+
+			if ( ! empty( $home_path_regex ) ) {
+				$req_uri  = preg_replace( $home_path_regex, '', $req_uri );
+				$req_uri  = trim( $req_uri, '/' );
+				$pathinfo = preg_replace( $home_path_regex, '', $pathinfo );
+				$pathinfo = trim( $pathinfo, '/' );
+				$self     = preg_replace( $home_path_regex, '', $self );
+				$self     = trim( $self, '/' );
+			}
 
 			// The requested permalink is in $pathinfo for path info requests and
 			// $req_uri for other requests.