Null data from XML

In case of reading data from XMl to Composite node following rule was
added:
- XML with opening and closing tag without data is mapped to single node
  with empty string
  (e. g. <foo></foo> -> single node with empty string)
- XML with only closing tag is mapped to single node with null value (e.
  g. <foo /> -> single node with null value)

Change-Id: I17ba4387949dd3cfe943999829dbcc63a29fbac8
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>