Nothing is a complicated concept in physics, for example, under one interpretation 'nothing is inherently unstable', and can flip into non-nothing. How this connects with the singularity, I don't know.
But in any case, some theists like to adopt it to suit their own arguments of creation 'ex nihilo', which produces a complete dog's breakfast of scientific ideas and theological ideas, not a good mixture.
(There is also the idea that the total energy in the universe amounts to zero, (combining positive energy with negative energy), but this is pretty indigestible stuff.)