[MacCatalyst] DatePicker null date handling (#31365)

### Description of Change

Created an UITest to validate the behavior and seems to be the same on
iOS MacCatalyst. When the DatePicker Date value is set to null and the
date picker is opened, the picker keeps today date.

## Expected behavior

- When DatePicker.Date is null, the MacCatalyst picker will display
today's date when opened
- The virtual view Date property remains null (as expected)
- Transitions between null and specific dates work correctly
- Existing functionality with non-null dates remains unchanged

### Issues Fixed

Fixes #31124

Author: jfversluis

src/Controls/tests/TestCases.HostApp/Issues/Issue31124.cs

@@ -0,0 +1,72 @@
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 31124, "DatePicker should maintain today's date when set to null", PlatformAffected.macOS)]
+public class Issue31124 : ContentPage
+{
+	DatePicker _datePicker;
+	Button _setNullButton;
+	Label _dateLabel;
+
+	public Issue31124()
+	{
+		InitializeComponent();
+	}
+
+	void InitializeComponent()
+	{
+		Title = "DatePicker Null Test";
+			
+		_datePicker = new DatePicker
+		{
+			AutomationId = "TestDatePicker",
+			Date = DateTime.Today
+		};
+
+		_setNullButton = new Button
+		{
+			Text = "Set Date to Null",
+			AutomationId = "SetNullButton"
+		};
+		_setNullButton.Clicked += Button_Clicked;
+
+		_dateLabel = new Label
+		{
+			Text = $"Current Date: {_datePicker.Date:d}",
+			AutomationId = "DateLabel"
+		};
+
+		// Listen for date changes to update the label
+		_datePicker.DateSelected += (s, e) =>
+		{
+			_dateLabel.Text = $"Current Date: {e.NewDate:d}";
+		};
+
+		Content = new VerticalStackLayout
+		{
+			Padding = new Thickness(20),
+			Spacing = 20,
+			Children =
+			{
+				new Label { Text = "Test: Setting DatePicker.Date to null should remove today's date" },
+				_datePicker,
+				_setNullButton,
+				_dateLabel,
+				new Label 
+				{ 
+					Text = "Expected: After clicking button, date should be null",
+					FontSize = 12,
+					TextColor = Colors.Gray
+				}
+			}
+		};
+	}
+
+	void Button_Clicked(object sender, EventArgs e)
+	{
+		// Set datePicker.Date to null
+		_datePicker.Date = null;
+			
+		// Update the label to show the current date
+		_dateLabel.Text = $"Current Date: {_datePicker.Date:d}";
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue31124.cs

@@ -0,0 +1,54 @@
+using System.Text.RegularExpressions;
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue31124 : _IssuesUITest
+{
+	public Issue31124(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "DatePicker should maintain today's date when set to null";
+
+	[Test]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerShouldMaintainTodaysDateWhenSetToNull()
+	{
+		// Find the initial date label text, should show today's date
+		var initialDateLabel = App.WaitForElement("DateLabel").GetText();
+		Assert.That(ContainsValidDate(initialDateLabel), Is.True, 
+			"DatePicker should initially show today's date");
+
+		// Click the button to set date to null
+		App.WaitForElement("SetNullButton").Click();
+
+		// Verify the date is empty after setting to null
+		var dateAfterNull = App.WaitForElement("DateLabel").GetText();
+		Assert.That(ContainsValidDate(dateAfterNull), Is.False,
+			"DatePicker should be empty after being set to null");
+	}
+	
+	static bool IsValidDate(string? input)
+	{
+		return !string.IsNullOrWhiteSpace(input) && DateTime.TryParse(input, out _);
+	}
+
+	static bool ContainsValidDate(string? labelText)
+	{
+		if (string.IsNullOrWhiteSpace(labelText))
+			return false;
+    
+		// Extract the date part after "Current Date: "
+		var prefix = "Current Date: ";
+		if (labelText.StartsWith(prefix))
+		{
+			var datePart = labelText.Substring(prefix.Length);
+			return IsValidDate(datePart);
+		}
+    
+		return false;
+	}
+}