[Xwt] Correctly handle a returned value of DBNull.Value
[Xwt] Correctly handle a returned value of DBNull.Value

If we get this value we should treat it as null and return the
defaultValue.

Prevents a typecastexception when trying to cast DBNull.Value to
'T'.

akrisiun/xwt,steffenWi/xwt,hamekoz/xwt,hwthomas/xwt,cra0zy/xwt,residuum/xwt,iainx/xwt,mminns/xwt,directhex/xwt,mminns/xwt,TheBrainTech/xwt,sevoku/xwt,mono/xwt,antmicro/xwt,lytico/xwt