ensure the MediaImporter and MediaExporter work with ImageCropper in U7.1.x

I am wondering if these few changes that I made would be any useful in the main branch, hence the pull request.

1. Updated the importer and exporter to work with ImageCropper in U7.1.x, which can be used to replace the Upload data type transparently in the backoffice.
2. Fixed the occassional exception thrown when property.ToXml() returns null.

Thoughts?