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.
It can't be `I*b`.  Imaginary numbers case handled right above `is_real is False`.