Fix statistics sensor mean and median when only one sample is available. (#11180)

* Fix statistics sensor mean and median when only one sample is available.

With only one data point stddev and variance throw an exception.
This would clear the (valid) mean and median calculations.

Separate the try..catch blocks for one-or-more and two-or-more stats so
that this doesn't happen.

Test this with a new sampling_size_1 test.

* test_statistics trivial whitespace fix

homeassistant/components/sensor/statistics.py

@@ -175,15 +175,20 @@ class StatisticsSensor(Entity):
             self._purge_old()
 
         if not self.is_binary:
-            try:
+            try:  # require only one data point
                 self.mean = round(statistics.mean(self.states), 2)
                 self.median = round(statistics.median(self.states), 2)
+            except statistics.StatisticsError as err:
+                _LOGGER.error(err)
+                self.mean = self.median = STATE_UNKNOWN
+
+            try:  # require at least two data points
                 self.stdev = round(statistics.stdev(self.states), 2)
                 self.variance = round(statistics.variance(self.states), 2)
             except statistics.StatisticsError as err:
                 _LOGGER.error(err)
-                self.mean = self.median = STATE_UNKNOWN
                 self.stdev = self.variance = STATE_UNKNOWN
+
             if self.states:
                 self.total = round(sum(self.states), 2)
                 self.min = min(self.states)