Fix crash in NAD integration

Some amplifiers/receivers do not report any volume (due to them not
knowing), for example NAD C 356BEE is documented to return an empty
string for volume.

At least don't crash when we get None back.
The type hint is optional int or float, since upstream is in the process
of adding float (some amps report floats back) and some report nothing.

Author: gladhorn

homeassistant/components/nad/media_player.py

@@ -1,5 +1,6 @@
 """Support for interfacing with NAD receivers through RS-232."""
 import logging
+from typing import Optional, Union
 
 from nad_receiver import NADReceiver, NADReceiverTCP, NADReceiverTelnet
 import voluptuous as vol
@@ -200,12 +201,16 @@ def update(self):
         self._volume = self.calc_volume(self._nad_receiver.main_volume("?"))
         self._source = self._source_dict.get(self._nad_receiver.main_source("?"))
 
-    def calc_volume(self, decibel):
+    def calc_volume(self, decibel: Optional[Union[int, float]]) -> Optional[float]:
         """
         Calculate the volume given the decibel.
 
         Return the volume (0..1).
         """
+        # Some receivers cannot report the volume, e.g. C 356BEE,
+        # instead they only support stepping the volume up or down
+        if decibel is None:
+            return None
         return abs(self._min_volume - decibel) / abs(
             self._min_volume - self._max_volume
         )