auth: GetLatestKey should allow NULL return values

auth: GetLatestKey should allow NULL return values

Existing callers in the code use
afsconf_GetLatestKey(dir, NULL, NULL) to check for the existence of
a key file. We need to permit NULL values for the return pointers
to this function.

Also update the tests to check for this behaviour.

Change-Id: I94e74138ddeed8d167c1e6f12e297411c638e1b9
Reviewed-on: http://gerrit.openafs.org/4050
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>