WButton is allowing 'whitespace only' text

[JohnMcGuinness]

If a WButton has text that is empty, as defined by Util.empty, and also does not have an image URL, accessible text or a tooltip, then an error should be thrown by WButtonRenderer.

The Util.empty method's java doc says it returns 'true if the given String is null or contains only whitespace'. This is incorrect as it only checks for white space that has a character code which is less than the ' ' (space) characters code. It ignores any whitespace that has a character code greater than that. There are many Unicode Whitespace characters with a code greater than the space characters character code.