Fix android perf bot issue caused by 1550803002

BUG=561190

Review URL: https://codereview.chromium.org/1553533002

Cr-Commit-Position: refs/heads/master@{#367097}

tools/telemetry/telemetry/internal/platform/power_monitor/sysfs_power_monitor.py

@@ -122,6 +122,7 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
         logging.warning(
             'Cannot read cpu frequency times for %s due to %s not existing'
             % (cpu, cpu_freq_path))
+        stats[cpu] = None
         continue
       try:
         stats[cpu] = self._platform.GetFileContents(cpu_freq_path)