[xaml] fix potential NRE in {Binding}

[jonathanpeppers]

Fixes: #24740
Context: cb0a332#diff-4f7a4722cd9e9622fcdb5c9c2a596997f8c6b7b9af8b51c5469e8ea59a3d9a48

Using this in a project:

<PackageReference Include="akgul.Maui.DataGrid" Version="4.0.4" />

Then, in a XAML file:

xmlns:dg="clr-namespace:Maui.DataGrid;assembly=Maui.DataGrid"
...
<dg:DataGrid />

Would crash with:

[0:] [13:46:57 FTL] Inner Exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Maui.Controls.Xaml.BindingExtension.<Microsoft.Maui.Controls.Xaml.IMarkupExtension<Microsoft.Maui.Controls.BindingBase>.ProvideValue>g__CreateBinding|40_0(<>c__DisplayClass40_0& ) in /_/src/Controls/src/Xaml/MarkupExtensions/BindingExtension.cs:line 46
at Microsoft.Maui.Controls.Xaml.BindingExtension.Microsoft.Maui.Controls.Xaml.IMarkupExtension<Microsoft.Maui.Controls.BindingBase>.ProvideValue(IServiceProvider serviceProvider) in /_/src/Controls/src/Xaml/MarkupExtensions/BindingExtension.cs:line 27
at Maui.DataGrid.DataGrid.InitializeComponent() in D:\Maui.DataGrid\Maui.DataGrid\Microsoft.Maui.Controls.SourceGen\Microsoft.Maui.Controls.SourceGen.CodeBehindGenerator\DataGrid.xaml.sg.cs:line 53
at Maui.DataGrid.DataGrid..ctor() in D:\Maui.DataGrid\Maui.DataGrid\DataGrid.xaml.cs:line 46
at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Constructor(Object obj, IntPtr* args)
at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Inspecting the IL, I see:

BindingBase val115 = ((IMarkupExtension<BindingBase>)(object)val31).ProvideValue((IServiceProvider)null);

So, it is indeed passed a null IServiceProvider.

Introduce a unit test and check for null as a fix.

[StephaneDelcroix]

will the legacy BindingExtension work ? if so, 👍