Nurse practitioner Martha Strong, asking Maryannie Flores, 7, to do her open-wide best, has operated Children's Clinic of Richardson for almost eight years. Nearly all of her 1,500 patients are enrolled in Medicaid. The shiny new clinic was quiet and nearly empty. It's been that way since Immaculata Inyang, a family nurse practitioner, opened the Potter's House Family Clinic last month in Oak Cliff. Six weeks, 17 patients. Still, Inyang is undaunted. She has no regrets plunking down her own money to start