ClipboardContent should throw NPE when null data passed

Native clipboard cannot handle null data. Currently we let users collect null references in ClipboardContent instance and later when it's passed to a Clipboard, user gets NPE. It would be more convenient to throw the NPE immediately from ClipboardContent, so that user gets NPE on the place where the null reference is passed instead of somewhere later (in which case he needs to search where the null reference was passed).