For me, in a general context the article is not needed because you can't count the rice. It is the same case with paella. Although, both cases can accept the article. Let me consider some cases. If you already mentioned it, the article is required to know that you are talking about the same rice. For example, Preparé arroz, ensalada y pescado. El pescado lo hice a la plancha, cociné el arroz con poca sal.
In additional, you have to be careful with the stress at the article el. If you say: cociné El arroz, it will be understood that your rice was very delicious or it has some unique feature. On the other hand, "cociné un arroz" or "cocine arroz" won't never have that meaning.