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.
That was the problem. We need to check if t is zero even if a None was already detected. e.g. Dummy(bounded=True)*Dummy(zero=True) is zero.