BUG-1425: fix LazyDataObject.toString()

A major usability improvement is skipping of null fields, as dumping
them adds quite a bit of verbosity to logs.

Change-Id: I94c030f11505718ee48b0e3f4556587a85f8f4e4
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1425: fix LazyDataObject.toString()"