when you draw a hazard card on green phase, sometimes they have 0. Does this mean you dont have to draw anything and win the round?
No. You must always draw at least one card. On p5, the rules say:
After you draw at least one fighting card against the hazard, you can decide to stop drawing additional cards, even if you do not draw all the free cards as written in the white box.
(bold as in the original)