What happens if the Queen and the striker are pocketed together and there are no other pieces on the board?
This can't happen.

From these rules, the Queen can't be left without a piece from each team still being on the board.

Pocketing either player's final piece with the Queen still on the board is a foul, which returns the pocketed piece and a penalty piece.

If the Queen and the striker were pocketed together while pieces were left, this is also a foul. Since a foul ends the turn, this means that the Queen can't then be covered, so it will return to the board.

However, from the ICF rules:

Somebody will already have won. Pocketing your own, or your opponent's final piece while the Queen is on the board causes you to lose.