[6.x] Improve folder empty states in asset browser

[duncanmcclean]

This pull request improves the folder empty state in the asset browser:

• Added top padding where we have bottom padding in Table/Listing mode.
• Added an empty state message in Grid mode.
• Ensured that the Grid & Table empty states share the same vertical padding, to avoid the message/card moving about when switching.

Fixes #12449