All use of `" null"` is replaced with `""` in an RFC's content

[bertdeblock]

Some examples:

• https://github.com/emberjs/rfcs/pull/875/files#diff-007eb250a8f0d75326107ccaa7b2047799e14adad7a9e4998df8e16f40f1bb6dR625
• https://github.com/emberjs/rfcs/pull/874/files#diff-13c3755669f99e14f510c8d659db8c77483173755382d5aa5e97be98058e4e2cR287
• https://github.com/emberjs/rfcs/pull/868/files#diff-5cd13663f87b395c0da8e155fc1e683f0aebc559ef4616731f02551481e845beR320

This happens here:
https://github.com/emberjs/rfcs-tooling/blob/main/lib/frontmatter-updater.js#L16

[kategengler]

Thanks for filing an issue! The replacement of null in that spot is because the frontmatter output turns any empty fields into null. This needs to run on just the frontmatter, not the entire doc.