I can't think of anything other than these two expressions "as if nothing" and "as if it were nothing" that mean what they mean, but I am not sure they mean the same thing, because I couldn't find them in a dictionary. I think "as if nothing" is an elliptical expression of "as if it were nothing", but is that really so?
For example:
The people in that town did a very weird thing, but they acted as if nothing.
"As if nothing" doesn't make sense here. You could say "...as if it were nothing" to make a statement about the "weird thing," or make a statement about "nothing" such as "...as if nothing were wrong." But not just "nothing" by itself.
Also, you might prefer to use "was" over "were," as it can sound more natural, although some prescriptivists would say "were" is slightly more correct. I'd say "were" sounds more natural after "it" than after "nothing."
"as if" is a phrase. There is no phrase such as "as if nothing" or "as if it was nothing". you can start a clause after "as if" by using "nothing". For example, he acted as if nothing has happened.
You may want to use the below sentence:
The people in that town did a very weird thing, but they acted as if it was nothing.
