Fix php offset error on empty string

Author: sebastianfeldmann

src/Util/Path.php

@@ -173,7 +173,7 @@ public static function withoutLeadingOrTrailingSlash(string $string) : string
     public static function isAbsolutePath(string $path) : bool
     {
         // path already absolute?
-        if ($path[0] === '/') {
+        if (substr($path, 0, 1) === '/') {
             return true;
         }