Update AuthenticationException.php

```$_SERVER['SERVER_ADMIN']``` might not always be set, especially if you are using nginx, instead of apache.

This produces a notice: 

```text
PHP Notice: Undefined index: SERVER_ADMIN in \CAS\AuthenticationException.php on line 77
```

I added the following logic, 
```php
if( isset($_SERVER['SERVER_ADMIN']) )
    return $_SERVER['SERVER_ADMIN'];
else
    return '';
```

This is how I simplified it:

```php
isset($_SERVER['SERVER_ADMIN']) ? $_SERVER['SERVER_ADMIN']:''
```