[mono.data.sqlite] Fixed the unit tests:
 - only write a column value if it is not an empty string (MonoTests.System.Data.DataSetTest.WriteXmlToStream)
 -- XmlTextWriter writes writer.WriteString("") as <element /> whereas XmlWriter.Create() writes it as <element></element>
 -- so, if the string is "", don't write it
 - line endings being generated