fix(VoiceConnection): handle nullable endpoint in voice server update payload

[amishshah]

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

Resolves #99.

Since a voice connection can now be in a disconnected state without a close code, the close code has now been made optional.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[codecov]

Codecov Report

Merging #100 (73873be) into main (686ef19) will increase coverage by 0.13%.
The diff coverage is 100.00%.

❗ Current head 73873be differs from pull request most recent head 6ecf755. Consider uploading reports for the commit 6ecf755 to get more accurate results

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   63.55%   63.69%   +0.13%     
==========================================
  Files          18       18              
  Lines         804      807       +3     
  Branches      185      187       +2     
==========================================
+ Hits          511      514       +3     
  Misses        291      291              
  Partials        2        2              

Impacted Files	Coverage Δ
src/VoiceConnection.ts	79.13% <100.00%> (+0.55%)	⬆️

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