fix: support setting null account sid

[aarya-brex]

Fixes

Account SID is a nullable field, we should allow setting the accountSid to null for users managing multiple rest clients

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[aarya-brex]

Will you be able to cut us a new version?

[childish-sambino]

Next release is scheduled for tomorrow (generally, every second Wednesday).