print no message at all when no argument is supplied to error()

this is useful when an adequate message has already been emitted by
other means, like various built-ins do.

Change-Id: I092771f55969fad8b214204d666327664727c572
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from qtbase/22d1351ddea193f5c00d4ae12229358dea826c62)