sleep_time_check.py failed to parse suspend/resume time

[Description]
sleep_time_check.py failed to parse

    # find our times
    for line in lines:
        try:
            if "Average time to sleep" in line:
                sleep_time = float(line.split(':')[1].strip())
                sleep_times.append(sleep_time)
            elif "Average time to resume" in line:
                resume_time = float(line.split(':')[1].strip())
                resume_times.append(resume_time)
        except ValueError as e:
            print("ERROR: One or more times was not reported correctly:")
            print(e)
            return 1

I guess the "Average time to sleep/resume" likely missing or showing unexpected output.
In the failure case, the checkbox submission doesn't include "pm_test.reboot.3.log".
Thus, we won't know what's going on.

[Summary]
power-management/suspend-30-cycles-time-check-with-reboots test failed

[Steps to reproduce]
Run checkbox-cli stress test

[Checkbox job `com.canonical.certification::power-management/suspend-30-cycles-time-check-with-reboots` output]

stdout
------
ERROR: One or more times was not reported correctly:
could not convert string to float: 'N/A'