Add null check

Author: vearutop

src/TextUI/Command.php

@@ -338,9 +338,10 @@ protected function handleArguments(array $argv)
                     break;
 
                 case '--test-suffix':
+                    $o1 = isset($option[1]) ? $option[1] : '';
                     $this->arguments['testSuffixes'] = explode(
                         ',',
-                        $option[1]
+                        $o1
                     );
                     break;