Richard A. Edwards
The rules say:
Strike: This icon allows a Troop Tile to capture from afar. Remove an enemy Troop Tile in one of the squares indicated by this icon (see Capturing Tiles, at right), but leave the Troop Tile in its original location; using this icon does not move the Troop Tile, but the Tile still flips
The rule says you can "capture" by removing an enemy Troop Tile.
Since an empty square doesn't have an enemy Troop Tile to capture, I'd say no, you cannot use Strike on an empty square.