Please allow the possibility to define empty macros if one desires to.

It it useful to have the ability to override a macro as empty especially when dealing with a link of templates.

In Zabbix 1.8, I used to set the macro to "" and that would work just fine. However, in Zabbix 2.0 I cannot do that any longer as the macro when expanded gets escaped "\"\" in the Zabbix Agent UserParameter.