Fatal error: Cannot use isset() on the result of an expression 

PHPSandbox throws an error if there's a check for a superglobal variable. 

The code:
```
<?php
isset($_SERVER);
?>
```

The error message:
```
Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in /.../vendor/corveda/php-sandbox/src/PHPSandbox.php(6885) : eval()'d code on line 8
```

