[AppKit] Allow NSGraphicsContext.CurrentContext to be null (#9842)

- Fixes https://github.com/xamarin/xamarin-macios/issues/9827

Co-authored-by: Chris Hamons <chris.hamons@xamarin.com>

src/appkit.cs

@@ -7160,7 +7160,7 @@ namespace AppKit {
 		[Deprecated (PlatformName.MacOSX, 10, 14, message: "Use 'FromCGContext' instead.")]
 		NSGraphicsContext FromGraphicsPort (IntPtr graphicsPort, bool initialFlippedState);
 	
-		[Static, Export ("currentContext")]
+		[Static, Export ("currentContext"), NullAllowed]
 		NSGraphicsContext CurrentContext { get; set; }
 	
 		[Static, Export ("currentContextDrawingToScreen")]

tests/xtro-sharpie/macOS-AppKit.ignore

@@ -1932,7 +1932,6 @@
 !missing-null-allowed! 'System.Void AppKit.NSFormCell::set_PlaceholderAttributedString(Foundation.NSAttributedString)' is missing an [NullAllowed] on parameter #0
 !missing-null-allowed! 'System.Void AppKit.NSFormCell::set_PlaceholderString(System.String)' is missing an [NullAllowed] on parameter #0
 !missing-null-allowed! 'System.Void AppKit.NSGestureRecognizer::set_Action(ObjCRuntime.Selector)' is missing an [NullAllowed] on parameter #0
-!missing-null-allowed! 'System.Void AppKit.NSGraphicsContext::set_CurrentContext(AppKit.NSGraphicsContext)' is missing an [NullAllowed] on parameter #0
 !missing-null-allowed! 'System.Void AppKit.NSGroupTouchBarItem::set_CustomizationLabel(System.String)' is missing an [NullAllowed] on parameter #0
 !missing-null-allowed! 'System.Void AppKit.NSHelpManager::FindString(System.String,System.String)' is missing an [NullAllowed] on parameter #1
 !missing-null-allowed! 'System.Void AppKit.NSHelpManager::OpenHelpAnchor(System.String,System.String)' is missing an [NullAllowed] on parameter #1