Do not allow minimum size drop to 0 because of population scale [Petra attack plan]

Petra reached maxed population and started spamming attack requests. Reported at forum https://wildfiregames.com/forum/index.php?/topic/27319-ai-chat-spamming/.

Cause of problem was that with given population scaling and current rounding minimal requested size of units dropped to 0.
What leaded to allow to start without units because minimum count have been reached.

This change requires at least one unit if minimum size is not 0 before adjustment by pop scaling , what we need to not block attacks with heroes or healers for example.

Now in case Petra is not able to produce units, aborts plan correctly.

Differential Revision: https://code.wildfiregames.com/D2462