Add Support for Nullable Date Values in DatePicker

[VidyalakshmiMani]

Description

The .NET MAUI DatePicker control currently does not support null values for its Date property, which limits its usability in scenarios where dates are optional. Many applications require fields that allow users to leave a date unset or clear a previously selected value, such as "Date of Birth" or "Event Date." This enhancement adds the ability for the DatePicker control to handle nullable DateTime values (DateTime?).

Public API Changes

1. New Property: AllowNull
   • Type: bool
   • Default: false
   • Description: Determines whether the DatePicker control allows the Date property to be set to null. When set to true, users can clear the date value, and the Date property can reflect null.

Intended Use-Case

1. Optional Date Fields:
   Forms often include optional fields such as "Date of Birth" or "Event Date," where the user should have the flexibility to leave the date unset or clear a previously selected value.

2. Reset Scenarios:
   Applications with reset functionality can use the AllowNull feature to clear date fields without additional code or custom logic.

[mattleibow]

Not sure we need a new property, we can maybe make the existing property nullable?

I guess that would be breaking because we would go from DateTime to Nullable<DateTime>...

Maybe we can add a new Value property that is nullable and even a DateOnly #1100

[mattleibow]

Duplicate of #1100