tests: avoid passing NULL strings to vprintf

tests: avoid passing NULL strings to vprintf

Some libc implementations will crash when NULL string arguments are given to
*printf.  Avoid passing NULL string arguments in the make check tests that did
so, and pass the string "(null)" instead.

Reviewed-on: https://gerrit.openafs.org/12377
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 2fe3a28c6ec0ff9d19ddec5500b3a5e69b483210)

Change-Id: Id8f1635444b5b49e3250addf36b64fccafd59941
Reviewed-on: https://gerrit.openafs.org/12396
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>