Improve code quality and test performance

[rameel]

Add VirtualNodeProperties.None:

Introduced a new static property VirtualNodeProperties.None to represent a node with no data instead of:

new VirtualNodeProperties(default, default, default, 0);
VirtualNodeProperties.CreateDirectoryProperties(default, default, default);

Optimize test performance:

Replaced CountAsync() with AnyAsync() in multiple test assertions where only the presence of items is checked. Since AnyAsync() short-circuits on the first element, this reduces enumeration overhead and improves test execution time.