[null-safety] fix type declaration of Picutre._toImage (#21942)

Fixes flutter/flutter#68377

Should be a nullable string. Looks like --null-assertions does not cover the native binding code.

lib/ui/painting.dart

@@ -4689,7 +4689,7 @@ class Picture extends NativeFieldWrapperClass2 {
     );
   }
 
-  String _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
+  String? _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
 
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.