Use true or false pcfgs, as appropriate, instead of null cfgs, when

Use true or false pcfgs, as appropriate, instead of null cfgs, when
generating disabled open-coded checks.  Although OPEN-CODE:WITH-CHECKS
filters out true pcfgs and null cfgs, these cfgs may be used directly
in pcfg predicates, as in OPEN-CODE:INDEX-CHECK, which permits only
pcfgs, not null cfgs.