Mapper: Allow automatic conversion to structs for null values

Versions 2.7.0 and 2.7.1 of the driver allowed by mistake autoconversions of null to default(T), being T an struct.
On v2.7.2, we introduced a change that causes the typing to be stricter. Even though this is the expected behaviour is a breaking change in for users that were relying in the driver to convert to default(T) when the value is null.

We should introduce a setting to allow users to continue using default values for structures to help transition to stricter typing (use nullable structs).