ktest.pl: Check kernelrelease return in get_version

ktest.pl: Check kernelrelease return in get_version

In the case of a test that uses the special option ${KERNEL_VERSION} in one
of its settings but has no configuration available in ${OUTPUT_DIR}, for
example if it's a new empty directory, then the `make kernelrelease` call
will fail and the subroutine will chomp an empty string, silently. Fix that
by adding an empty configuration and retrying.

Cc: stable@vger.kernel.org
Cc: John Hawley <warthog9@eaglescrag.net>
Fixes: 5f9b6ced04a4e ("ktest: Bisecting, install modules, add logging")
Link: https://lore.kernel.org/20241205-ktest_kver_fallback-v2-1-869dae4c7777@suse.com
Signed-off-by: Ricardo B. Marliere <rbm@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>