Fix handling of null "standalone" value.

As part of transitioning the OS towards FastXmlSerializer, we found
at least one example of a caller who is omitting a "standalone" value
by passing null.  Handle this by mirroring the behavior of the
upstream KXmlSerializer and omitting the attribute entirely when
the value is undefined.

Bug: 174561276
Test: manual
Change-Id: I868aceaed8e3dd1575947a56b28f8fff8759c0ae