Temporary revert previous patch

Split previous patch in 2 steps: first remove
the MOVE_NULL hack, then retire nullChild.

The first step is a prerequisite
for second one and affects bench.

The second step (next patch) just removes nullChild
without affecting bench.

bench: 8205159
Remove 'update gains' hack

Use (move != MOVE_NONE) condition to
filtering out updating gains at root.

bench: 8454456
Avoid to use nullChild

Use instead:

(ss-1)->currentMove == MOVE_NULL

No functional change.