fix is_zero detection on Mul._eval_is_* tests

Once a None is obtained for is_zero, it should no longer be updated.
The return value must consider the value of is_zero. This can
change if S.Zero.is_imaginary = S.Zero.is_real = True is ever true.

The routines were shortened a bit, too.
check `if zero is False` instead (and you can simplify if-elif below).  If zero=None - we don't want to go in this branch.