You should create your own meaning and purpose in life. If this does not happen you will most likely be sad.

I don't claim to be a philosopher but I would call that realism. Also being happy all the time is ridiculous, without sad there cannot be happy.

Atheism has nothing to do with nihilism.

Or you could tell the person that nihilism has nothing to do with Atheism, and ask him/her why he/she thinks it does.

Actually I would simply argue that the act of being atheist does not merit their worthless floccinaucinihilipilification

floccinaucinihilipilification' “The action of malignly estimating some thing, some action or some state of existence as being worthless"

IDK about nihilism but actually, without GOD my life is more meaningful and purposeful.