Less_ImportVisitor: Fix "Using null as an array offset" PHP 8.5 warning

In less.js, they do something like "fullPath in importVisitor.recursionDetector" which is kind of the equivalent to array_key_exists. We would also still encounter this error if we use array_key_exists because less.js also passes in null to the function. These values are set in ImportManager.prototype.push and if you narrow down, the actual callback is ImportVisitor.prototype.onImported.

Bug: T411400
Change-Id: I4ad03834f87e01ae316494fab1f8205480722f46

Author: anny21

lib/Less/ImportVisitor.php

@@ -164,7 +164,7 @@ public function processImportNode( $importNode, $env, &$importParent ) {
 				throw $e;
 			}
 
-			$duplicateImport = isset( $this->recursionDetector[$fullPath] );
+			$duplicateImport = $fullPath && isset( $this->recursionDetector[$fullPath] );
 
 			if ( !$env->importMultiple ) {
 				if ( $duplicateImport ) {