Allow empty styles at runtime

For dynamic styles/rules we need to allow them render empty in order to be linked and so props can be defined later. Right now the logic allows empty rules only if link: true option is used, which works fine.

Though, a more straight forward way would be to always allow empty rules. We need to skip empty rules only when we do SSR or build-time extraction in order to remove unused CSS.

Empty rules could be generated when a function which returns styles, but, based on some condition, returns an empty value and user is unaware. In general it is a quite rare case and gave us headaches with dynamic cases where rules are generated by Observables and evtl. by functions in the future.