fix: NameFilter returning null causes Map keys to serialize as null (#7644)

[1919chichi]

What this PR does / why we need it?

When NameFilter.process() returns null for a Map entry key in ObjectWriterImplMap.writeWithFilter(), the key is directly overwritten with null and serialized as null:"value" instead of preserving the original key name.

Fixes #7644

Summary of your change

Added a null guard in ObjectWriterImplMap.writeWithFilter() so that when NameFilter.process() returns null, the original key is preserved — consistent with the existing behavior in:

• ObjectWriterAdapter (POJO fields): boolean nameChanged = filteredName != null && ...
• JSONObject.nameFilter(): if (processName != null && !processName.equals(key))

Before:

if (nameFilter != null) {
    key = nameFilter.process(object, key, value); // null overwrites key directly
}

After:

if (nameFilter != null) {
    String filteredKey = nameFilter.process(object, key, value);
    if (filteredKey != null) {
        key = filteredKey; // null → preserve original key
    }
}

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[wenshao]

Fix is correct and minimal. The null guard in ObjectWriterImplMap.writeWithFilter() properly preserves the original Map key when NameFilter.process() returns null, consistent with the existing behavior in JSONObject.nameFilter(). Tests cover the core scenarios well. LGTM! ✅

— qwen3.7-max via Qwen Code /review

[wenshao]

No issues found. LGTM! ✅

— Qwen Code via Qwen Code /review