ST. LOUIS — Inside the more than 600 Catholic hospitals across the country, not a single nun can be found occupying a chief executive suite, according to the Catholic Health Association. Nuns founded and led those hospitals in a mission to treat sick and poor people, but some were also shrewd business leaders. Sister Irene Kraus, a former chief executive of Daughters of Charity National Health System, was famous for coining the phrase “no margin, no mission.” It means hospitals must succeed — generating enough revenue to exceed expenses — to fulfill their original mission. The Catholic Church …