Unless you are joining in as a co-founder or a founding team member with significant equity in play from the get go, I would say evaluate it based on conviction but even then I don't believe paying nothing justifies it. You have to make bread to at least be in a sustainable position.
As a deterrent, check with the founders that in case of no funding would they be able to pay the individual a basic amount on the work done by them. And also, how would the salary structure and arrear look like post fundraising, do they cover it well after that ?
If the answer to the above is either dicey or deflective in nature, avoid it because they just want free work from you without the conviction to give you back anything for 6 months of your work life.