Disallow deleting a blank tile

Steps to recreate:

1. Open 'Select Tile' window'
2. Click a blank tile
3. Press delete key
4. Observe "Confirm Delete" Yes/No msgbox.

This can create the confusing impression that the tile isn't actually blank. Consider having trying to delete a blank tile display the "Remove Tiles" msgbox instead.

That msgbox is created by delete_tiles in zq_tiles.cpp