Avoid null

[jwes]

Crafty (and probably other engines) avoid null move if the hash table has a fail low with depth >= to null depth and score < beta. I believe that these conditions are too restrictive. I ran a quick test to see how often null moves would give cutoffs for all fail lows in the hash table that did not meet the other conditions and found that the cutoff rate was ~ 1%. This is much less than the cost of doing a null move, so the program should run faster (and slightly more accurately) by avoiding null move anytime that the hash table has a fail low.