Change: Don't require the to prop for Link component

Actually it is optional and it is set to an empty string by default
already.

src/web/components/link/link.jsx

@@ -82,7 +82,7 @@ Link.propTypes = {
   anchor: PropTypes.string,
   filter: PropTypes.oneOfType([PropTypes.filter, PropTypes.string]),
   query: PropTypes.object,
-  to: PropTypes.string.isRequired,
+  to: PropTypes.string,
 };
 
 Link = styled(withTextOnly(Link))`