mac 12.4 crashed

```bash
$ psrecord 49119 --plot ~/prof.png --duration 60
Attaching to process 49119
Traceback (most recent call last):
File "/Users/jeffery/Library/Python/3.8/bin/psrecord", line 7, in
sys.exit(psrecord.main())
File "/Users/jeffery/Library/Python/3.8/lib/python/site-packages/psrecord/main.py", line 103, in main
monitor(pid, logfile=args.log, plot=args.plot, duration=args.duration,
File "/Users/jeffery/Library/Python/3.8/lib/python/site-packages/psrecord/main.py", line 219, in monitor
ax.set_ylim(0., max(log['cpu']) * 1.2)
ValueError: max() arg is an empty sequence

```

Reproduce the reported `psrecord 49119 --plot ~/prof.png --duration 60` command on macOS 12.4 and inspect `psrecord/main.py`, especially `monitor` around line 219 where the empty CPU sequence reaches `max()`. Confirm the expected behavior when no CPU samples are recorded and verify that the command no longer crashes.

Written by the indexing model from the issue text.