Fix str_starts_with() null deprecation in get_current_url_supercache_dir

[davidcarma]

Summary

• Fixes a PHP 8.1+ deprecation warning: str_starts_with(): Passing null to parameter #2 ($needle) of type string is deprecated on line 857 of wp-cache-phase2.php
• The global $wp_cache_home_path can be null when not defined in the cache config, which gets passed directly to str_starts_with() and rtrim() as a null needle/characters parameter
• The fix coalesces $wp_cache_home_path to an empty string and skips the prefix check entirely when the home path is empty, avoiding both the deprecation and the semantically incorrect rtrim(null, '/') call

Reproduction

1. Run any WP-CLI command or trigger a WP-Super-Cache operation on PHP 8.1+ where $wp_cache_home_path is not set in the cache config
2. Observe the deprecation notice:

   Deprecated: str_starts_with(): Passing null to parameter #2 ($needle) of type string is deprecated
   in /var/www/html/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 857
   

Changes

-  if ( ! str_starts_with( $uri, $wp_cache_home_path ) ) {
-    $uri = rtrim( $wp_cache_home_path, '/' ) . $uri;
+  $home_path = $wp_cache_home_path ?? '';
+  if ( $home_path !== '' && ! str_starts_with( $uri, $home_path ) ) {
+    $uri = rtrim( $home_path, '/' ) . $uri;
   }

Notes

• Related but different from PR Check for null string before escaping #848, which addressed a null $wp_cache_request_uri passed to esc_url_raw() on line 394
• This is a minimal, targeted fix with no behavioural change when $wp_cache_home_path is properly set

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