BTRoot : set actor_path from script instead of Inpesctor cause underlying _actor to be always null

[Earewien]

Scenario:

• create a BTRoot, and do not assign the actor path from Inspector
• in script file, add root as a child of another node in scene tree
• assign BTRoot actor_path from script

This result in a null actor at runtime

Analysis:
In BTRoot, the underlying _actor variable is updated when actor_path changes in inspector (from update_warnings), or in ready function when checking node validity.
Changing actor_pass after _ready function has no effect. In actor_path setter, underlying _actor should be updated.