[Petra/AI]Handle 0 case for template parsing

[Petra]Check against undefined when caching variables from templates.

Current check does not allow to read 0 and false values from templates and represents them as undefined.

Forum link: ​https://wildfiregames.com/forum/topic/29206-questions-on-ai-api3-template-handling/
Reported by: Teiresias
Patch by: smiley
Reviewed by: Angen
Differential Revision: D3010