There are some times in Ascension when I managed to empty my deck by drawing all the cards in it and also all the cards in the discard piles. It happened once or twice that I played a card that makes me draw another card. Are there special rules to cover this case, or do I simply ignore the drawing effect of the card?
You simply skip the draw part of the instruction.
According to Justin Gary, one of the designers of the game:
If you have to draw a card and none are available in your deck or discard pile, you simply skip the draw.