Move emptySpace() check to isValidMove().

This eliminates a null reference access in the case where there is no
ChessPiece at the oldRow/oldColumn position.
Remove forceMove() method.

The forceMove() method had some flaws that I couldn't fix. I can't
properly update the  internal location of a piece if it moves outside
its normal movement pattern. I deliberately chose to exclude setters for
ChessPiece's internal location so they couldn't move wrongly.
Trim trailing whitespace.