Is Zero zero?

[projekter]

I just found out that curiously, iszero(Zero()) is not defined. Is this on purpose or just an oversight?
(Basically, this comes from @blegat's suggestion that imag on a real-valued AbstractVariable should return Zero - checking if an AbstractPolynomialLike has zero imaginary part therefore breaks if it is a variable.)

[blegat]

We didn't eagerly define all methods we could find, we're just waiting for use cases in order to define methods for Zero. This looks like a reasonable use case so PR welcome