Are conditional line-up clearances still valid if the referenced traffic initiates a go-around / missed approach?
This may sound obvious, but you should probably ask ATC.

In the sentence given it specifies that you should line up behind the Landing B757, not the Going Around B757. This sounds pedantic, but the situation has changed.

The chances are the ATC is still expecting you to be the next aircraft on the tarmac, but if you're in any doubt whatsoever, do not put your aircraft on a live runway (or anywhere else you can reasonably expect another aircraft to want to be in the next 5 minutes) without explicit, unambiguous permission.