Allow Deleting of List if they only have 1 list left / Make sure the Products stay within the array

I found some basic issues with the bronto api

1. If a user is only on one list, the request would not send null to force them to be removed 
2. Products wouldn't be saved within the array since there was no public variable to store them in

I'm not following this addition. If the `listIds` field is empty, add a null value to the `listIds` array field?
```
bash-3.2$ php -r '$test = array(); $test[] = null; var_dump($test);'
array(1) {
  [0]=>
  NULL
}
```
Can you describe what the 309-315 addition is doing?