I ran into the same problem and the suggested converter solution doesn't seem to work either. I can insert a dummy value for emptyness, but that is not a good solution at all. After I thought some time about it I came to the conclusion that this behaviour is probably not intended (because serializing/deserializing doesn't return an equal object), so maybe it should be corrected at the core, hence filing this bug.
It is described here: http://stackoverflow.com/questions/37099126/simple-framework-ignores-empty-xml-elements-in-list
I ran into the same problem and the suggested converter solution doesn't seem to work either. I can insert a dummy value for emptyness, but that is not a good solution at all. After I thought some time about it I came to the conclusion that this behaviour is probably not intended (because serializing/deserializing doesn't return an equal object), so maybe it should be corrected at the core, hence filing this bug.