fix empty areaToModify variable if block is added to a global area

Fixes http://www.concrete5.org/developers/bugs/5-7-2/concretecorepermissionkeyaddblockblocktypekey-does-not-have-a-me/

The error occurred when a block was being added to a global area. The $this-> areaToModify variable remained empty if the default Area name "Main" also didn't exist in the template.