[Super Cache] Fix str_starts_with() null deprecation in get_current_url_supercache_dir (#47645)

Co-authored-by: Brandon Kraft <public@brandonkraft.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/24094045575

Upstream-Ref: Automattic/jetpack@234d523

Author: kraftbj

CHANGELOG.md

@@ -20,6 +20,7 @@ This is an alpha version! The changes listed here are not final.
 ### Fixed
 - Ensure proper flags are used with `json_encode()`.
 - Fix TS errors detected by tsgo.
+- Fix `str_starts_with()` PHP 8.1+ deprecation when `$wp_cache_home_path` is null in `get_current_url_supercache_dir()`.
 
 ## [3.0.3] - 2025-11-11
 ### Added

wp-cache-phase2.php

@@ -853,9 +853,10 @@ function get_current_url_supercache_dir( $post_id = 0 ) {
 				$uri = '';
 			}
 		} else {
-			$uri = str_replace( $site_url, '', $permalink );
-			if ( ! str_starts_with( $uri, $wp_cache_home_path ) ) {
-				$uri = rtrim( $wp_cache_home_path, '/' ) . $uri;
+			$uri       = str_replace( $site_url, '', $permalink );
+			$home_path = $wp_cache_home_path ?? '';
+			if ( $home_path !== '' && ! str_starts_with( $uri, $home_path ) ) {
+				$uri = rtrim( $home_path, '/' ) . $uri;
 			}
 		}
 	} else {