This fixes the bug: https://bugzilla.proxmox.com/show_bug.cgi?id=677

Remarks:
* allowBlank is already set in the class definition
* The value of the comboBox can be either '' after the component initial
load, or 'null' if the user selected a macro and cleans the field afterwards.
This is the reason of the if (!value) test which will catch empty strings and
null.