Verify for null before calling setVolume

[Nansess]

Changes

Prevents null TypeError.

Why

To ensure a stream is playing/cached before trying to modify its volume.

Checkmarks

• [X] The modified endpoints have been tested.
• [X] Used the same indentation as the rest of the project.
• [X] Still compatible with LavaLink clients.

[ThePedroo]

Hello, thank you for the PR.

Even though I like it, it doesn't comply with our standards. Could you fix them according to our contributing repository so that I can merge it?