[Appkit] Adds NullAllowed to NSTableCellView.ObjectValue (#3793)

Fixes xamarin/xamarin-macios#3792

`null` is allowed in `NSTableCellView.ObjectValue` property according to header definition:

```
@property (nullable, strong) id objectValue;
```

src/appkit.cs

@@ -16431,7 +16431,7 @@ namespace AppKit {
 			get; set;
 		}
 
-		[Export ("objectValue", ArgumentSemantic.Retain)]
+		[Export ("objectValue", ArgumentSemantic.Retain), NullAllowed]
 		NSObject ObjectValue {
 			get; set;
 		}