fix(CNavGroup): set default `height` state to 0

[jasperfirecai2]

Closes https://github.com/coreui/coreui-react/issues/410 The root cause of the issue is that height is undefined while the transition state is exiting and thus the style sets height to undefined. Then when the transition starts on opening the dropdown, the height is instantly set to auto meaning there's no startpoint to transition from. This fix is a suggestion. This can also be solved using props or more complex style conditions.