NEW YORK — For Shannon Henderson, getting a cold or flu could be the difference between putting food on the table and going hungry.As a part-time customer service representative at a Wal-Mart in Sacramento, Calif., Henderson is one of an estimated 40 million American workers for whom calling in sick is a luxury. If they don’t work, they don’t get paid.“I’m super afraid of getting sick,” said Henderson, 29, who slathers on hand sanitizer at work in hopes of fending off illness.Paid sick …