fixup! perf test: Add truncated perf.data robustness test

Guard "${perfdata}.old" removal against empty variable — when the first
mktemp fails, perfdata is empty and "${perfdata}.old" expands to ".old",
which would delete an unrelated file in the current directory.

Reported-by: sashiko-bot <sashiko@sashiko.dev>
Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>