feat: Do not serialise empty custom headers array

[Frederick888]

Description

36ce219 feat: Do not serialise empty custom headers array

There are some unexpected side effects according to [1], where other unrelated headers like In-Reply-To, References, etc. were purged. This minimises the blast radius.

[1] https://github.com/Frederick888/external-editor-revived/issues/110

Checklist

• [x] I have rebased my branch so that it has no conflicts
• [ ] I have added tests where appropriate
• [x] Commit messages are compliant with Conventional Commits

Is this a breaking change?

No

Test results

• OS: Linux
• Thunderbird version: 102