Fix media_player supported features default value (#33366)

I looked at all media_player components and these were the only ones returning None:
- bluesound
- emby
- mpd

homeassistant/components/bluesound/media_player.py

@@ -777,7 +777,7 @@ class BluesoundPlayer(MediaPlayerDevice):
     def supported_features(self):
         """Flag of media commands that are supported."""
         if self._status is None:
-            return None
+            return 0
 
         if self.is_grouped and not self.is_master:
             return SUPPORT_VOLUME_STEP | SUPPORT_VOLUME_SET | SUPPORT_VOLUME_MUTE

homeassistant/components/emby/media_player.py

@@ -305,7 +305,7 @@ class EmbyDevice(MediaPlayerDevice):
         """Flag media player features that are supported."""
         if self.supports_remote_control:
             return SUPPORT_EMBY
-        return None
+        return 0
 
     async def async_media_play(self):
         """Play media."""

homeassistant/components/mpd/media_player.py

@@ -249,7 +249,7 @@ class MpdDevice(MediaPlayerDevice):
     def supported_features(self):
         """Flag media player features that are supported."""
         if self._status is None:
-            return None
+            return 0
 
         supported = SUPPORT_MPD
         if "volume" in self._status: