feat(typeaccessor): add isnull custom implementation

[DeagleGross]

expected implementation:

public override bool IsNull(T obj, int index) => index switch {
    1 or 2 or 6 => false, // non-nullable value-type
    3 => obj.Name is null, // known reference-type
    4 => obj.DateOfBirth is null, // nullable value-type
    5 => obj.Token is null or DBNull, // object : consider null and the special DBNull
    7 => true, // known to be DBNull ... so: can only ever be null or DbNull, so: always null!
    _ => throw...
}

[DeagleGross]

@mgravell the only thing I dont understand how to check is num 7 from your example: I cant think of a scenario, where property can only be null or DBNull. Can you please assist with explanation?

Other cases are covered I think

[mgravell]

public DBNull StupidProperty {get;set;}

Is completely pointless, but can only be null or DBNull. In reality, this is so overwhelmingly stupid that perhaps I'm overthinking and .StupidProperty is null or DBNull is fine (just like object), but ... meh

[DeagleGross]

let me add the check as well for that stupid case

[mgravell]

Nice, thanks