Fix warning on PHP 7.4

Created by: PierrickVoulet

7.4 comes with a warning when trying to access array offset on value of type int.

This is to fix one condition that generates some of these warnings.

src/Arguments.php

@@ -99,7 +99,7 @@ class Arguments
      */
     protected function isValue($arg)
     {
-        return (empty($arg) || $arg === '-' || $arg[0] !== '-');
+        return (empty($arg) || $arg === '-' || 0 !== strpos($arg, '-'));
     }
 
     /**