Return type of the property "volume" should be Optional (#172)

* The return type of "volume" is optional now. The Air Purifier 2 doesn't support the property.

* Example payload updated.

miio/airpurifier.py

@@ -29,23 +29,23 @@ class AirPurifierStatus:
 
     def __init__(self, data: Dict[str, Any]) -> None:
         """
-        Response of a Air Purifier Pro:
+        Response of a Air Purifier Pro (zhimi.airpurifier.v6):
 
-        ['power': 'off', 'aqi': 41, 'humidity': 62, 'temp_dec': 293,
-         'mode': 'auto', 'led': 'on', 'led_b': null, 'buzzer': null,
-         'child_lock': 'off', 'limit_hum': null, 'trans_level': null,
-         'bright': 71, 'favorite_level': 17, 'filter1_life': 77,
-         'act_det': null, 'f1_hour_used': 771, 'use_time': 2776200,
-         'motor1_speed': 0]
+        {'power': 'off', 'aqi': 7, 'average_aqi': 18, 'humidity': 45,
+         'temp_dec': 234, 'mode': 'auto', 'favorite_level': 17,
+         'filter1_life': 52, 'f1_hour_used': 1664, 'use_time': 2642700,
+         'motor1_speed': 0, 'purify_volume': 62180, 'f1_hour': 3500,
+         'led': 'on', 'led_b': None, 'bright': 83, 'buzzer': None,
+         'child_lock': 'off', 'volume': 50}
 
-        Response of a Air Purifier 2:
+        Response of a Air Purifier 2 (zhimi.airpurifier.m1):
 
         {'power': 'on, 'aqi': 10, 'average_aqi': 8, 'humidity': 62,
          'temp_dec': 186, 'mode': 'auto', 'favorite_level': 10,
         'filter1_life': 80, 'f1_hour_used': 682, 'use_time': 2457000,
         'motor1_speed': 354, 'purify_volume': 25262, 'f1_hour': 3500,
         'led': 'off', 'led_b': 2, 'bright': None, 'buzzer': 'off',
-        'child_lock': 'off'}
+        'child_lock': 'off', 'volume': None}
 
         A request is limited to 16 properties.
         """
@@ -150,7 +150,7 @@ def motor_speed(self) -> int:
         return self.data["motor1_speed"]
 
     @property
-    def volume(self) -> int:
+    def volume(self) -> Optional[int]:
         """Volume of sound notifications [0-100]."""
         return self.data["volume"]