NORTHAMPTON —  History isn’t certain whether Napoleon or Frederick the Great of Prussia coined the phrase “An army marches on its stomach,” but the meaning itself is clear: An armed force is a lot more effective when it’s well-fed. It might be a stretch to say the economy marches on its stomach, but the health of the economy is often reflected in the restaurant business. According to the Massachusetts Restaurant Association, about 300,000 people — some 10 percent of the state’s workforce — are employed in serving food and beverage.