Verify COMPOSER_VENDOR_DIR in conditional

This corrects an issue where `COMPOSER_VENDOR_DIR` could be an empty value and this condition erroneously runs. Before running this condition we need to confirm `COMPOSER_VENDOR_DIR` is set and contains a value.

For further details on this issue see:

- craftcms/cms#6773
- laminas/laminas-feed#22

src/Autoloader.php

@@ -66,7 +66,7 @@ public static function load()
      */
     private static function getClassLoader()
     {
-        if (file_exists(getenv('COMPOSER_VENDOR_DIR') . '/autoload.php')) {
+        if (getenv('COMPOSER_VENDOR_DIR') && file_exists(getenv('COMPOSER_VENDOR_DIR') . '/autoload.php')) {
             return include getenv('COMPOSER_VENDOR_DIR') . '/autoload.php';
         }