fix(Voice*): filter out silent audio from video users

[amishshah]

Please describe the changes this PR makes and why it should be merged:

Problem:
This resolves #5019. When a user has video enabled, they send silent audio frames continually even if they are not speaking. Discord.js didn't check whether a received audio packet was silence or not for performance reasons. This shortcut is no longer applicable since it causes the incorrect behaviour described in the issue above.

Fix:
Instead of decoding all the received packets, we only decode the packets from users with video enabled (which is where the issue occurs). If a packet is received from such a user and it turns out to be silent audio, the packet is ignored which is a graceful fix for this issue. This means performance will only degrade (and even then, I doubt it will be by a lot) in cases where a bot is in the same voice channel as users with video enabled, and the bot is actively receiving audio.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.