IAT payload value is not respected if value is zero #874

Description

If the iat payload value is is zero, in other words, the UNIX epoch, it is not respected and the current time is used instead. I believe it is because zero evaluates to false here:

https://github.com/auth0/node-jsonwebtoken/blob/master/sign.js#L179

Reproduction

jwt.sign({ iat: 0, data: 'foobar' }, 'secret', { expiresIn: 86400 });

Expected payload iat value should be zero and exp is expected to be 86400. The actual iat value is the current time and exp is the current time plus 86400.

Environment

• Version: 9.0.0
• Node v14.18.1 (npm v8.12.2)