Add parameterless constructor to TreeView component

[Copilot]

Overview

This PR adds a parameterless constructor to the TreeView component class, allowing developers to create a TreeView instance without immediately assigning items to it.

Changes

TreeView.cs

Added a new parameterless constructor that follows the same pattern used by other components in the toolkit (e.g., DropDown):

/// <summary>
/// Initializes a new instance of the <see cref="TreeView" /> class.
/// </summary>
public TreeView() : this(Enumerable.Empty<TreeViewItem>())
{
}

This constructor chains to the existing parameterized constructor with an empty enumerable, ensuring proper initialization of all internal fields.

Usage Example

Before this change, developers had to provide items when creating a TreeView:

var treeView = new TreeView(new[] { item1, item2 });

Now, they can create an empty TreeView and populate it later:

var treeView = new TreeView();
// ... later in the code
treeView.Items = new[] { item1, item2 };

Testing

Added a comprehensive unit test TreeViewParameterlessConstructor() that verifies:

• TreeView can be instantiated without parameters
• The Items collection is properly initialized (not null) and empty
• Items can be assigned after creation
• All TreeView functionality works correctly with items added post-construction

Security

CodeQL security analysis completed with 0 alerts found. No security vulnerabilities were introduced.

Fixes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>TreeView constructor without items</issue_title>
<issue_description>Add an extra constructor to the TreeView component class, that allows to create a treeview without immediately assigning items to it.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #45

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