fix Issue 11730 - associative array with Nullable!SysTime values: Called `get' on null Nullable!SysTime

When I fixed bug 6178, I internally used `cast(void)exp)` to suppress unexpected "has no side effect" error.

But, `CastExp` always try to resolve alias this of its operand to find most matching type to `void` via `Expression::castTo`. It should be disabled.