The compiler does not currently have any specific support for nullable types, though this could certainly be a useful enhancement. It does have support for the use of implicit and explicit conversions and System.Nullable<T> defines an implicit conversion from type T to System.Nullable<T>. This means that you can replace the set statement in your example by:
set testDecimal to 0 as decimal
...and the correct conversion code will be generated.
I will investigate better support for nullable types for a future compiler release.