Only loop the Zoo Visitor Member Categories if they exist. 

Prevents the following PHP errors: 

A PHP Error was encountered
Severity: Warning
Message: Invalid argument supplied for foreach()
Filename: models/mailchimp_model.php
Line Number: 1056

A PHP Error was encountered
Severity: Notice
Message: Undefined property: Addons_extensions::$_zoo_visitor_member_categories
Filename: core/Model.php
Line Number: 50