Make sure no early move generation in null-move reply

Make sure no early move generation in null-move reply

Now that we explicitly test for pinning of the e.p. victim, early move
generation (to test for King capture) is only needed after King moves.
But the toPiece on which we test this can be undefined after null move.
Rather than explicitly testing for null move when setting earlyGen,
we set toPiece to a Pawn when making the null move.