ktest: Avoid undef warning when WARNINGS_FILE is unset

check_buildlog() probes $warnings_file with -f even when WARNINGS_FILE is
not configured. Perl warns about the uninitialized value and adds noise to
the test log, which can hide the output we actually care about.

Check that WARNINGS_FILE is defined before testing whether the file exists.

Cc: John Hawley <warthog9@eaglescrag.net>
Cc: Andrea Righi <arighi@nvidia.com>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Fernando Fernandez Mancera <fmancera@suse.de>
Cc: Pedro Falcato <pfalcato@suse.de>
Link: https://patch.msgid.link/20260307-ktest-fixes-v1-1-565d412f4925@suse.com
Fixes: 4283b169abfb ("ktest: Add make_warnings_file and process full warnings")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>