WEST PALM BEACH, Fla. — For the past few days, Jack Nicklaus has been sick, listing a bit, even telling his oldest son he might need to lean on him. As he pulls his gold Lexus onto the tarmac of Signature Flight Service, he’s still considering canceling this trip. He thinks of all the times he showed up when he didn’t feel like it. He even kept an appointment after his grandson died; a devastated Jack believed a commitment should be honored. That’s how his dad taught him to act. No, pushing 70, he can’t remember ever missing a business trip. He doesn’t want to start now, so he’s here, on muscle memory mostly, fighting a sinus infection and a hacking cough, parking his car next to the gleaming white Gulfstream V — tail number N1JN, affectionately called Air Bear — but even now, he isn’t sure. It’s not too late to turn around.