Bug fixes.
 -The string BclClass return true for its IsNullable property.
 -A string property definition is not excluded because there hasn't been a non-empty value for the property yet.
 -When emitting the setter for the property code for a NullableBclClass, set the backing property's value to null if the 'value' setter parameter is empty.
Properties are indexed by a DomPath instead of their element name.