Fix FileNode crash on terminator / unknown node types (baseType == 2)

[twortley]

Problem

FileNode.__init__ raises AttributeError: 'FileNode' object has no attribute 'data' on some valid .one files, aborting the whole parse.

It happens for any FileNode whose header baseType == 2 (a reference node) but whose file_node_id isn't in _FileNodeIDs (so file_node_type resolves to "Invalid"). The dispatch chain's final else branch (p = 1) never sets self.data, and the next block then dereferences self.data.ref for reference nodes.

The real-world trigger is a FileNodeListFragment terminator. The fragment loop breaks on file_node_id in (0, 255) — but only after the node is constructed:

while file.tell() + 24 < end:
    node = FileNode(file, document)   # constructs first (crashes here)
    self.fileNodes.append(node)
    if node.file_node_header.file_node_id in (255, 0):
        break                         # terminator check happens too late

So a terminator whose upper header bits encode baseType == 2 crashes during construction, before the loop can break. Larger / heavily-revised sections hit this; small files usually terminate cleanly.

Fix

Default self.data = None for unknown node types, and only recurse into a child FileNodeList when self.data is not None. Known reference nodes are unaffected, and the fragment loop's existing terminator-break then ends the fragment cleanly.

Test

Adds a self-contained regression test: a 4-byte FileNode header (id=0, baseType=2) reproduces the crash with no sample .one file needed.

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Found while building an offline OneNote→Markdown exporter — this crash blocked parsing of several large real notebooks. Thanks for pyOneNote!