An empty moment could simply mean a quiet waking moment not "filled" with activities. Alternatively, it could refer to the Buddhist concept of emptiness. To master the emptiness mode of perception would require firm training in virtue, concentration, and discernment. Without this training, the mind stays in the mode that keeps creating stories and worldviews. And from the perspective of that mode, the teaching of emptiness sounds simply like another story or worldview with new ground rules. In terms of your worldview, it seems to be saying either that the world doesn’t really exist, or else that emptiness is the great undifferentiated ground of being from which we all came and to which someday we’ll all return.
empty hours - having no purpose or result https://www.merriam-webster.com/dictionary/empty Moments in life lacking emotion, interest, or purpose https://www.macmillandictionary.com/dictionary/american/empty_1
If you describe a person's particular period of time as empty, you mean that nothing interesting or valuable happens in it. If you feel empty, you feel unhappy and have no energy, usually because you are very tired or have just experienced something upsetting. https://www.collinsdictionary.com/dictionary/english/empty
It likely means a time when nothing of import, consequence, or interest was happening.
I would expect that "empty moment" in context meant a time in your life when all seemed meaningless, perhaps due to the loss of someone close; a life once fulfilled is now empty of something held dear..