Fix take sound on non captures

[johndoknjas]

We must check if the dest square has a piece before making the move.

[ornicar]

what does this fix?

Isn't pieceFormerlyAtDest null in case of en-passant?

[johndoknjas]

@ornicar Good point about en passant, I'll fix that.

Currently took is true even on non-capture moves, since our own piece has already moved to the dest (so checking if there's a piece on the dest is meaningless). E.g. for this, every move makes the take sound.