pts: Don't malloc(0) when there's nothing to do

pts: Don't malloc(0) when there's nothing to do

If GetNameOrId is called with no work to do, then don't attempt to
malloc a load of 0 length strings. Instead just return an empty array
to the caller.

Change-Id: I245cfde71d65b8a3b6df4217b90dad81e9e60a58
Reviewed-on: http://gerrit.openafs.org/7100
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>