LPS-51165 Don't create empty <dynamic-element>, it causes xml validation failure

Fix for StagingImplTest

CC @marcellustavares

I don't really know this, but I feel it should be the way to fix it.

Basically, DDMFormXSDSerializerImpl generates an empty <dynamic-element> and saved into db, later when we read it back, it breaks on DDMXMLImpl.validateXML(String)