Do not fill the healthbar for negative values

At the moment, if you have 0 hp, the healthbar is empty. But if you enter negative values for the actual hp, the healthbar gets filled again.

For example: If you have -5/10, the healthbar gets half filled.

This is very confusing, especially if you have disabled the numbers that are displayed in the healthbar. It would be nice and much less confusing if the bar would remain empty for negative values.