Ignore empty replay path + hide a global (to avoid init fiasco)

Ignore empty replay path + hide a global (to avoid init fiasco)

I was observing that the model_checking_mode global did not had the
same value when compiling with or without optimizations.

Maybe I had a sort of race condition on model_checking_mode during
initialization, or maybe the config element of the replay path was
either called before or after the initialization of the library, thus
replacing the model_checking_mode to REPLAY even if it was supposed to
be APP_SIDE in my case.

It's fixed now, but I'm not completely sure of which of these change
was the right one (probably the replay path). Both seem legit, so commit both.