Just backing what Twist said, Morgana's ult is not a channel.
A mage who cant zhonya's with an ult would be Anivia or Karthus, their ultimates are a Channel. As such any interruption (stuns, silences, and including zhonya's) will cancel the pool/laser.
This is different from toggled abilities which only turn off if you cast it again or if you run out of mana. (Someone silencing you just means you can't turn it off.)
I don't know how Zhonya's interacts with her ability, it's likely it will cancel it because it acts as a self-silence, but I haven't used it on her in ages (since it was Zhonya's Ring).