fix: unable to position in center if first step has no DOM node

[dimitar-nikovski]

In the latest version if the first step is intended as a modal in the center of the screen and no DOM selector is passed, the Guide renders at the top left of the screen because it doesn't know the dimensions of the helper and the props passed to the Guide styled-component produce NaN px transform values.

[MarcLopezAvila]

Please fix this