fixed separateCommas() on empty string

Running String.split() on empty string produces an array with one -- empty string -- element (```[ "" ]```). This leads to the following problem:

Setting the ```nics.*.allowed_ips``` to empty array with ```vmadm update``` will successfully set the zone property to empty string, but the output of ```vmadm get``` is not reusable and for example a consecutive ```vmadm send/receive``` will fail with *invalid allowed_ips*.