Shape-Reader: Do not create a property if its value is null

[JohannaOtt]

Version 4.0.0.SNAPSHOT Build 640814825

When using a Shapefile as source schema and importing source data, the view in the SchemaExplorer indicates that there are values for all attributes for all objecs in the dataset (according to numbers in brackets). But when checking the data in the source data view, for some objects, some of the attributes are null (and for other objects and other attributes, there are empty strings - the behavior is the same for all of them). That means, currently the Shapefile reader created a property always - independently from if this property has the value null.

It would be helpful if this behavior would be changed in a way that the Shapefile reader does not create a property if the value is null (instead of creating it with a null value). This would be more aligned to the behavior of the XML reader, where no property is created if a property is not there for an object.