channel is nullable if disconnecting (#553)

Author: vim2meta

docs/resources/Voice.md

@@ -9,7 +9,7 @@ Used to represent a user's voice connection status.
 | Field | Type | Description |
 |-------|------|-------------|
 | guild_id? | snowflake | the guild id this voice state is for |
-| channel_id | snowflake | the channel id this user is connected to |
+| channel_id | ?snowflake | the channel id this user is connected to |
 | user_id | snowflake | the user id this voice state is for |
 | session_id | string | the session id for this voice state |
 | deaf | bool | whether this user is deafened by the server |