fix(serialization-form): omit the root object assignment prefix

[RKS (rksharma-owg)]

Serializing a root form object with write_object_value(None, model) produces =body=example instead of body=example. The writer omits the absent key but still emits its assignment separator.

Only add the key and = when a key is supplied. Root objects now contribute their encoded fields directly. Empty roots leave existing output untouched; named empty objects still emit their key and assignment separator. Named-object serialization and field escaping retain their existing behavior.

Validation (Python 3.12):

• Five regressions failed before the change: absent/empty root keys, an existing field, merged object values, and an empty root object.
• Review regression: empty roots after an existing field failed for both absent and empty-string keys before the follow-up fix. Added a named-empty-object compatibility check.
• Form package: pytest -q — 53 passed.
• YAPF, isort, mypy, and git diff --check passed.
• Pylint exited successfully with 10/10; it also reports the existing unrecognized suggestion-mode configuration option.

Closes #476.

Prepared with AI assistance; reproduction and validation ran locally.

[Vincent Biret (baywet)]

Thanks for the contribution!

[Vincent Biret (baywet)]

Thank you for making the changes!

[Copilot]

🟢 Approval recommended

Validation confirms the fix and preserves existing named-object behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud