fix(map): improve null field serialization and deserialization

[Joltras]

Proposed changes

The map serialization and deserialization now handles fields with null values correctly. This prevents null fields from being lost or causing issues when converting maps between their serialized and deserialized representations.

Types of changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[github-actions]

Test results

204 files  204 suites   1m 36s ⏱️
239 tests 235 ✅  4 💤 0 ❌
720 runs  708 ✅ 12 💤 0 ❌

Results for commit 4c785c7.