Default values of Any AI Behaviour Tree node Blackboard Key should be NULL / Unhandled

The default value of Blackboard Key params for newly added service, decorator or task should be some sort of NULL value which is detectable in a runtime or at least editor only Unhandled key which would produce warnings in an attempt to run a tree with such node.

Right now it defaults to first key on the blackboard, which makes copy-pasting and creating new b tree nodes error prone.