No, because it is not in Earth orbit
First the payload does have a purpose: it is a boilerplate, and those have a purpose, namely to "test various configurations and basic size, load, and handling characteristics of rocket launch vehicles".
Second, you are asking...
is the car equipped with a propulsion system to change its trajectory in case of imminent collision risk ?
No, it is not. The payload is not in Earth orbit any more. It is in an elliptical heliocentric orbit. The likelihood that is will ever be a collision risk for anyone or anything is infinitesimal.
Generally — as this NASA page states — we only consider objects in Earth orbit to possibly be space junk, or "orbital debris" as the more technical term for such things are.
1). What are orbital debris?
Orbital debris are all man-made objects in orbit about the Earth which no longer serve a useful purpose.
The reason only objects in Earth orbit are considered "orbital debris" is because only those are of relevance to us. We do not expect to ever run into man-made objects that are not in Earth orbit, simply because the probability of a collision is so small and the number of them is so low, that combined it is not worth the effort to try to prevent any such collisions. By comparison we do not even try to protect against meteor/meteorite strikes even though the Earth is hit by such — the size of the Roadster or larger — several times each year. If that does not bother us enough to warrant taking measures to prevent it from happening, why would the Roadster warrant it? It simply does not.
If you personally want to call the Roadster "space junk" you may do that of course, but I will counter that by saying "Yeah but it is harmless space junk".