add a check for minOccurs == 0 when determining member nillable property

Currently `Variable.nillable` is set to true only when the wsdl element has the `nillable` attribute set to `true`. 
However this is not the only way (nor the typical way) of making fields accept null values. The "normal" way of making fields accept null values is by setting the `minOccurs` attribute to `0`.
See http://www.ibm.com/developerworks/webservices/library/ws-tip-null/index.html
I think `null` should be returned here as well?