[MacCatalyst] DatePicker null date handling

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

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+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

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+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;
+	}
+}