RECYCLEBINEMPTY is a special condition built in to Opus for exactly this purpose. But you can also test for things like variables set by commands and scripts.
Remove the FORCE argument from the last line if you'd like to see a confirmation dialog before anything happens (assuming you have Windows set to confirm emptying the recycle bin).
Add the QUIET argument to the last line if you'd like to prevent any progress dialog while the recycle bin is being emptied.
Tip: When editing a button which uses @icon, hold Ctrl and click where it says @icon to open an image picker, so you don't have to manually work out the path and icon ID.
There is no need. The icon of the recycle bin is automatically set according to the status of the recycle bin. The status of the recycle bin can be easily seen from the icon. This is the biggest feature of this button.