Handle MAKE-ACTION for actions with empty preconditions

[rpgoldman]

make-action seems to emit a nil for empty preconditions. There seem to be two ways to handle this:

1. simply don't emit the preconditions -- according to the PDDL spec, the preconditions are optional.
2. emit a preconditions form that is true in every state. I believe that this would be (and), but need to verify.

Whatever we choose, need to verify it works on the most popular PDDL planners and on validate.