Correct Undefined Index error in mail.class.php

When senderName is not defined crons fail resulting in Undefined Index error on line 59 in mail.class.php. 
```php
$aData['senderName'] = $senderName;
```
To temporarily fix this added (line 35):
```php
if (empty($senderName)) { $aData['senderName'] = $senderEmail; }
```