✨ Add nullable key to needs_fields items

[chrisjsewell]

This pull request introduces support for a nullable property in need field definitions, allowing fields to be explicitly marked as allowing or disallowing unset (None) values. The changes affect the configuration, schema validation, and documentation, and include updates to both the codebase and the test suite to ensure correct handling and enforcement of field nullability.

For example:

needs_fields = {
   "introduced": {
      "description": "Date when the need was introduced",
      "schema": {
         "type": "string",
         "format": "date",
      },
      "nullable": False,
   },
}

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.24%. Comparing base (4e10030) to head (975e42a).
⚠️ Report is 221 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/needs_schema.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1613      +/-   ##
==========================================
+ Coverage   86.87%   88.24%   +1.36%     
==========================================
  Files          56       70      +14     
  Lines        6532     9929    +3397     
==========================================
+ Hits         5675     8762    +3087     
- Misses        857     1167     +310     

Flag	Coverage Δ
pytests	88.24% <97.50%> (+1.36%)	⬆️

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[ubmarco]

How would users know the default for nullable? This depends on the field, we should state this somewhere.

[chrisjsewell]

Obviously, this is only for inherited core fields, but yes we should show their "full" configuration in the needs_fields documentation.
Maybe leave this for a follow-up PR?

[chrisjsewell]

Done https://sphinx-needs--1613.org.readthedocs.build/en/1613/configuration.html#core-field-specialization

[ubmarco]

LGTM, thanks for the update