Fix caller_path in the case of nil
Fix caller_path in the case of nil

`Thread::Backtrace::Location#absolute_path` can return nil so it would
raise exception NoMethodError with `include?` when it returns nil.

PikachuEXE/bullet,rrosenblum/bullet,flyerhzm/bullet,rrosenblum/bullet,flyerhzm/bullet,flyerhzm/bullet,PikachuEXE/bullet