No check for `id != 0` in many functions

[adhusson]

Several functions will accept the id 0. It is better to add a 0 check in _validateId (or better: don't check id at all and check vesting.receiver != address(0)).

It would not only match the behavior of DssVest, it would prevent can setting bits on the vesting with id 0 using protect for instance:

vest.protect(0);
Vest.Vesting memory vesting = vest.getVesting(0);
assertTrue(vesting.protected); // works

[MerlinEgalite]

Good idea!