null guard in `loadOrSave`

Currently setting a null `val()` to a tinymce jquery (e.g. `$(':tinymce').first().val(null)`) will case an unhandled crash.

This is perfectly legal in jquery ([fails silently](http://james.padolsey.com/jquery/#v=1.7.2&fn=jQuery.fn.val)), but ed.setContent doesn't allow null values to be passed through, so we need to guard against that.