CNavGroup: First time opening dropdown pops items into existence instead of playing transition animation

[jasperfirecai2]

When you first open a dropdown item in a sidebar nav, the items instantly appear instead of playing the height transition animation. Subsequent openings work as intended. If you manually add height: 0px in the browser debugger before opening the dropdown, the transition plays as expected.

Bug reports must include:

• Operating system and version: Windows 11 64-bit & WSL 2
• Browser and version: Firefox latest
• Test case: https://coreui.io/react/docs/components/sidebar/#sidebar-component open the dropdown in the example sidebar of the docs. Suggested fix would be defaulting the height state to 0.