The fact that the Voyager spacecraft continues to travel out of the solar system, even though its rockets have no fuel, is an example of

Newton's First Law of motion

Explanation:

Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. The Voyager Spacecraft continues to travel (without acceleration) means that there is no resultant force acting on the voyager that will accelerate / decelerate the spacecraft.

If the Voyager were accelerating even though it ran out of Fuel, it would've been due to the gravitational force of attraction of celestial bodies. It would've been an example of The universal law of gravitation which states that every particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.