Handle the case freq state data read from file is empty for sysfs_power_montior.

Breakage caused by this case can be found in http://build.chromium.org/p/chromium.perf/builders/Android%20Nexus4%20Perf/builds/1416/steps/page_cycler.bloat/logs/stdio

BUG=

Review URL: https://codereview.chromium.org/610483002

Cr-Commit-Position: refs/heads/master@{#296861}

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

@@ -162,7 +162,7 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
     for cpu in initial:
       current_cpu = {}
       total = 0
-      if initial[cpu] is None or final[cpu] is None:
+      if not initial[cpu] or not final[cpu]:
         cpu_stats[cpu] = collections.defaultdict(int)
         continue
       for state in initial[cpu]:

tools/telemetry/telemetry/core/platform/power_monitor/sysfs_power_monitor_unittest.py

@@ -16,7 +16,8 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
             '1200000 2082\n1100000 2943\n1000000 6560\n900000 12517\n'
             '800000 8690\n700000 5105\n600000 3800\n500000 5131\n400000 5479\n'
             '300000 7571\n200000 133618',
-    'cpu2': '1700000 1131'
+    'cpu2': '1700000 1131',
+    'cpu3': '1700000 1131'
   }
   final_freq = {
     'cpu0': '1700000 7159\n1600000 0\n1500000 0\n1400000 68\n1300000 134\n'
@@ -27,7 +28,8 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
             '1200000 2272\n1100000 3163\n1000000 7039\n900000 13800\n'
             '800000 9599\n700000 5655\n600000 4144\n500000 5655\n400000 6005\n'
             '300000 8288\n200000 149724',
-    'cpu2': None
+    'cpu2': None,
+    'cpu3': ''
   }
   expected_initial_freq = {
     'cpu0': {
@@ -68,6 +70,9 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
     },
     'cpu2': {
       1700000000: 1131
+    },
+    'cpu3': {
+      1700000000: 1131
     }
   }
   expected_final_freq = {
@@ -107,7 +112,8 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
       300000000: 8288,
       200000000: 149724
     },
-    'cpu2': None
+    'cpu2': None,
+    'cpu3': {}
   }
   expected_freq_percents = {
     'whole_package': {
@@ -166,7 +172,10 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
     },
     'cpu2': {
       1700000000: 0.0,
-    }
+    },
+    'cpu3': {
+      1700000000: 0.0,
+   }
   }
 
   def testParseCpuFreq(self):