Exceptions in converts for PadFoundation and PileFoundation

[pawelbaran]

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.