hgweb: simplify the handling of empty repo This abstraction have two advantages: - If the revlog is empty, None of the code bellow is relevant, early returns seems a win. - Abtraction of the 'emptiness' check will help later when we stop relying on nodefunc. A bonus, with filtering, a non-empty revlog may not have '0' revision accessible. It'll be easier to handle with the emptiness test in a dedicated function