Do not razor after a null move

Do not razor after a null move

We don't want to return unproven null move fails high, so
that if a position is so good that null move fails high we
want to check this with real do_move() / undo_move() test,
not just razoring the position because, from the opponent
point of view, is very bad.

These are tests results at 1+0

Mod vs Orig +252 -264 =483  49.40%
Mod vs Toga II 1.4.1SE  +365 -325 =309  52.00%

So it seems a very slightly regression regarding orig version (but
withing error bar) and a nice increase against Toga that is what we
are interested most. Orig version scores 49.75% against Toga, so
we welcome this change ;-)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>