One symbol is the absence of the cafe. In other words, this absence represents nothing or "nada" as it is said in the story. The old man seems to have nothing. His wife is gone; his only solace is having a drink in the cafe. Without the cafe, there is nothing for him. The old waiter understands the empty feeling this absence instills; the younger waiter has yet to understand or experience this in life. And according to the old man and older waiter, the other bodegas don't provide the same kind of empathetic solace that the cafe does. So, this notion of the absence of the cafe, nada, is an experiential symbol of loneliness and maybe even chaos -- since the cafe provides some order for the old man, a light in the nothing.
The old man has attempted suicide because he was in despair about "nothing." He is deaf so he can hear nothing. The light itself, more welcoming than the bodegas, is a "something" in the "nothing." Light is a common symbol in literature and here it could symbolize simple presence (of warmth, humanity), order, truth, and life (death symbolizing the light going out).
The old waiter's prayer filled with "nada" and "nothing" is an attempt to make sense of nothing, to give it order and structure and meaning in the structure and spiritual significance of a prayer. Likewise, the cafe and the light in particular give some sense of structure and meaning, even empathy (to the old man). He goes there every day, like a ritual meeting. And that is the symbolism of the prayer itself: just as the old man seeks empathy in the cafe/light, the old waiter seeks empathy (someone else to understand him) with his mocking attempt at a prayer, another ritual. The ritual itself is symbolic as an attempt to find meaning. The waiter concludes his night with more searching for empathy. "After all, he said to himself, it is probably only insomnia. Many must have it." He reaches out to the "many." The need for empathy is, for the old man and the older waiter, a reaching out for others as well as an escape from the loneliness of (having) nothing.