As mentioned before, the TCA is neutral on the question of whether the universe had a beginning or not. In fact, Thomas adamantly denies that the universe's beginning could be demonstrated by reason alone (although he did affirm it by faith). Whatever one's thoughts on this are, the Biblical teaching is that the universe did begin to exist (Gen. 1:1) and that God created it out of nothing (creatio ex nihilo).
Now, if out of nothing comes nothing (ex nihilo nihil fit), then how did God create the universe out of nothing? One might argue that this is an impossibility. Could a Thomistic perspective on metaphysics benefit us in any way? I think so.
An actuality is something that is real. A potentiality, on the other hand, is something that is not real but could be real. For example, an acorn is merely an acorn in actuality, but in potentiality it is an oak tree.
This deserves some additional consideration. Even though I just said that potentialities are not real, in another sense they are quite real. After all, if the potentiality for an acorn to become an oak tree does not exist, then in what sense is it possible for an acorn to become an oak tree? We might revise our earlier statement by saying that potentialities are not real-qua-actualities, but that potentialities are real-qua-metaphysical-possibilities.
Imagine now the state of affairs in which God exists without the universe. The potentiality for the universe's existence is real-qua-metaphysical-possibility, so God's creative act, or actualization, of the universe does not entail creation out of nothing per se. Rather, it entails creation out of nothing already actualized.
Maybe this viewpoint is persuasive, and maybe not. Nonetheless, the above does allow the theist to affirm both creatio ex nihilo and ex nihilo nihil fit, since the former allows for potentialities to be real-qua-metaphysical-possibilities and the latter only denies that something could come from nothing in a more absolute sense.