Emergency responders in Bucks County Friday searched the Delaware River for a body seen floating near the Frenchtown Bridge earlier in the week.   Investigators believe it could be a 62-year-old man that was reported missing.   His car was apparently found in a parking lot near a ball field not far from the bridge.   Six rescue boats combed an eight mile stretch of the river between Nockamixon and Point Pleasant but found nothing.   A search conducted by two police K-9 units on shore also came up empty.