Is it possible to sacrifice a Monster character while Nyogtha is ready and has no wounds? 

Nope. If the intent had been to allow this, the ability would have been written similar to this:

Sacrifice a monster. Then ready Nyogtha or remove one of his wound tokens.

 Thank you!

=)

 Hello again, 

while i was reading the FAQ, I found this:

(2.13) Multiple Effects and the word “Then”
Some cards have effects that attempt to do more than one thing. Generally, these effects resolve independently of one another. (If the first effect of the card does not or cannot resolve, the second effect will still occur.)
For example: Y’Golonac’s (Core Set F122) ability reads: “Action: pay 1 to choose and ready a character. That character must commit to the same story as Y’Golonac, if able.” Y’Golonac can target a ready character with this ability, and even though the first part (ready a character) does not resolve, the second part of the effect does resolve.

as it says, you can pay the cost even if the consequence cannot take place (the character is ready)

so I don't see why I couldn't do the same with Nyogtha… 

and in this other situation, could I drain my domain and put a success token if my opponent can't drain on of his domains? (I coun't it as 0 resources, but maybe it's not like this…)

 There have been past rulings that when there is no numerical value, it is not treated as 0, but rather as "does not compute".  So in your Blackball jim example, you would not have more resources than your opponent - you would have an incomputable answer.

Thank you =)

what about the possibility to trigger Nyogtha's action without wounds and readied?

 The issue comes in with the "to" part of the description.  You don't have a viable target (A ready character can't be readied, and a character without wound tokens can't have a wound token removed).  So there is nothing to do.  If there is no target, you can't start the effect.  Sorry!

 But if you see the FAQ (the part I just copied above), Y'Golonac can choose a ready character to "ready" it…

Glendora said:

Y'golonac's action have two effects.

•    First effect: Pay 1 to choose and ready a character
• Second effect: That character must commint to he same story as Y'golonac (if able)

You can choose a ready character, and then first effect fails but second effect can be played.

Nyogtha have only one effect (although it have an "or")

 I'm gaining uncertainty.  But I think the ultimate answer is indeed as marcoa explains - Y'Golnac is trying to do 2 things.  So we can have 1 fail if the other happens, and the payment still is required.  Nygotha has only a single effect, and if nothing can happen then you can't even try.

My uncertainty is because Y'Golnac has 2 sentences rather than one…

 I wanted yo sacrifice the Living Mummy to trigger its effect using the ability of Nyogtha

sometimes happened that Nyohtha was ready and had no wounds, so I did not know if I could sacrifice the mummy anyway

 I dont see why you couldnt.

Action: Sacrifice a Monster character to ready Nyogtha, or to remove a wound token from Nyogtha.

Ready Nyoga has no targeting requirements that be violated by nyogtha being ready or anything like that. I see no reason not to trigger it to do nothing, you would only pay the cost of sacrifice.

Any rules fragment that forbids it?

With other things that actually cannot be triggered there are targetting problems like not enough cards or Invulnerable cannot be wounded and similar, i see nothing of that sort here.

Hmm. I'm reluctant to make a call, right now, however, I'd like to point out that interpretation would mean you could use Nyoghta repeatedly to sacrifice any number of (monster) characters every turn. Considering how that would interact with certain Silver Twilight cards, I doubt that's how it was intended.