gh-100710: Add nodeValue default to xml.dom.minidom.Node base class

[SomSamantray]

Fixes gh-100710.

Node.nodeValue is documented per the DOM interface it implements, but the base Node class in Lib/xml/dom/minidom.py never declares it — only concrete subclasses (Element, DocumentFragment, DocumentType, Entity, Notation, Document as a class attribute; Attr, ProcessingInstruction, CharacterData via a property) set it.

At runtime this is harmless since every instantiable subclass already sets nodeValue. The reported symptom is static analysis: code holding a Node-typed reference (e.g. iterating childNodes typed as Node) and accessing .nodeValue triggers a false-positive "unknown member" error from type checkers like Pylance, because the attribute isn't declared on the base class.

Fix

Add nodeValue = None as a class-level default on Node, matching the existing pattern already used for namespaceURI, parentNode, ownerDocument, nextSibling, and previousSibling.

Testing

• Added testNodeValueDefaultOnBaseNode in Lib/test/test_minidom.py, asserting the base Node class (and a bare Node() instance) exposes nodeValue as None.
• Ran the full test_minidom suite locally (132 tests, all passing).

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[SomSamantray]

@python-cla-bot check

[serhiy-storchaka]

Node is practically an abstract class and is not intended to be instantiated, so Node() in the test creates something which should not exist.

The addition itself is fine: it matches the other defaults on Node (parentNode, ownerDocument, prefix), and allows removing nodeValue = None from DocumentFragment, Element, DocumentType, Entity, Notation, EntityReference and Document.

But nothing changes at runtime, so there is nothing to announce in NEWS. And the reported problem is already solved for type checkers: typeshed declares nodeValue: str | None on Node.

Please remove the test and the NEWS entry. The redundant definitions in the subclasses also can be removed -- nodeValue in the seven classes above, and also parentNode in DocumentFragment and Document, and previousSibling and nextSibling in Document, which already repeat the defaults of Node.