fix(voice): mark stream as ended

[nyapat]

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

closes #8778

example:

• subscribe to user.
• user starts speaking (pipeline is: check if subscription exists, if stream exists, voice packets are decrypted, and pushed to stream)
• user stops speaking (specifically when endbehavior aftersilence or afterinactivity determines its 'stopped', push null to mark end)
• user starts speaking (subscription exists: yep, stream exists: yep, push to stream!)
• error: pushing after stream EOF

Status and versioning classification:

• 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