The NMPlatformIP[46]Address and NMPlatformIP[46]Route structs have a
field 'dev'. Before this field was always printed in the *_to_string
functions and a missing device was signaled as ' dev -'.
This had the advantage, that the output contained the same fields
regardless whether there was a device set or not.

Change it, not to print the device if it is not set. This has the
advantage, that it looks better in the logfiles.

Signed-off-by: Thomas Haller <thaller@redhat.com>