As I understand it, this question is not asking how Genesis 1:1 (or other Biblical passages) are interpreted today, but rather, how such passages were interpreted anciently.
There are Christians today who read these passages and understand them to describe creatio ex nihilo, and there are Christians today who read these passages and understand them to describe creatio ex materia. Other posts on this site address contemporary views; discussions on Mi Yodeya address Jewish history. I'll focus here on where we find these ideas in early Christian history.
The earliest surviving Christian source to clearly describe creatio ex nihilo is Tatian, writing in the 2nd half of the 2nd century:
And as the Logos, begotten in the beginning, begat
in turn our world, having first created for Himself the necessary matter, so also I, in imitation of
the Logos, being begotten again, and having become possessed of the truth, am trying to reduce
to order the confused matter which is kindred with myself. For matter is not, like God, without beginning (Oratio ad Greacos ch. 5)
Irenaeus of Lyons, writing a few years after Tatian, may be described as the first major/mainstream Christian writer to argue explicitly for creatio ex nihilo (Tatian was rejected by contemporaries, including Irenaeus, as a heretic--see Irenaeus Against Heresies 1.28.1).
Creatio ex nihilo is not found in the extant writings of the Apostolic Fathers (2nd generation Christians, writing ~AD 70-120), nor in the writings of Tatian's teacher Justin Martyr (major works ~AD 150-165), who argued for creatio ex materia:
God, having altered matter which was shapeless, made the world, hear the very words spoken through Moses, who, as above shown, was the first prophet, and of greater antiquity than the Greek writers; and through whom the Spirit of prophecy, signifying how and from what materials God at first formed the world, spoke thus: In the beginning God created the heaven and the earth. And the earth was invisible and unfurnished, and darkness was upon the face of the deep; and the Spirit of God moved over the waters. And God said, Let there be light; and it was so. So that both Plato and they who agree with him, and we ourselves, have learned, and you also can be convinced, that by the word of God the whole world was made out of the substance spoken of before by Moses (1st Apology ch. 59).
In the generation prior to Tatian the extant Christian sources all favor creatio ex materia. In the generation following Tatian there are Christian writers arguing for creatio ex nihilo (e.g. Irenaues, Origen of Alexandria) and there are Christian writers arguing for creatio ex materia (e.g. Hermogenes, Clement of Alexandria). Creatio ex nihilo became a more dominant viewpoint by the end of the 3rd century.
Further reading on the relevant history here.
