Bug 8982: avoid .transform() NPEs

Bug 8982: avoid .transform() NPEs

Guava’s Optional::transform doesn’t support null values returned by
the mapping function — it throws a NPE if that happens. java.util’s
Optional::map does support null values and returns an empty Optional.
Combined with the current YANG-based code’s ability to return null,
this means it’s better to use java.util’s method; this patch switches
all YANG-based Optional transforms to java.util.

Change-Id: I304762b8eed03480760d36ae6903cdb495bd7b05
Signed-off-by: Stephen Kitt <skitt@redhat.com>