Fix TypeError on round(self.humidity) (fixes #13116) (#14174)

* Fix TypeError on round(self.humidity)

Some weather platforms postpone the first data fetch for a while on init. As a result round(self.humidity is called before it is assigned a value, producing an error. This is a fix for that.

* Rewrite to avoid false negative evaluation

As per the suggestion from @OttoWinter, rewrite to avoid matching e.g. 0.0 as false.

homeassistant/components/weather/__init__.py

@@ -113,9 +113,12 @@ class WeatherEntity(Entity):
             ATTR_WEATHER_TEMPERATURE: show_temp(
                 self.hass, self.temperature, self.temperature_unit,
                 self.precision),
-            ATTR_WEATHER_HUMIDITY: round(self.humidity)
         }
 
+        humidity = self.humidity
+        if humidity is not None:
+            data[ATTR_WEATHER_HUMIDITY] = round(humidity)
+
         ozone = self.ozone
         if ozone is not None:
             data[ATTR_WEATHER_OZONE] = ozone