fix perf-stats without specifying events

eval.py

@@ -79,7 +79,7 @@ def main(args):
             perf_out = RESULT_DIR / f'{target}.perf.data'
             cmd = f'{PERF_EXE} record --call-graph dwarf -o {perf_out} {cmd}'
 
-        elif args.perf_stats or args.perf_record:
+        elif args.perf_stats is not None or args.perf_record:
             perf_event_selection = ','.join(
                 args.perf_stats) if args.perf_stats else f'-e {PERF_EVENTS}'