Exceptions in converts for PadFoundation and PileFoundation

#### Description:

#1579 introduced a more precise mechanism to distinguish between different types of foundations in `BHoMType` method, which means that for a typical pile, it returns `typeof(Pile)`, and accordingly for pad and pile foundations. That works well. However, the improvement causes an exception downstream because `ConvertMethod` does not find correspondent convert methods for `PileFoundation` and `PadFoundation` because they simply do not exist. So we need a temp fix that will make `BHoMType` return `null` for nonexistent types until they get properly implemented.