Only consider kingless positions as in-check in Atomic

Only consider kingless positions as in-check in Atomic

The check test returned 1000 (as special version of TRUE) when there
was no King that could be exposed to capture, under the assumption that
this would mean it had been exploded by the preceding move. But this was
counter-productive in variants without King (such as Horde). So now it
is only done in Atomic Chess, and all other variants return FALSE if there
is nothing on the board that could be checked.