Cute. Except I'm not asking anyone to prove a negative. Then I would have said "prove that NONE of the Church Fathers believed this."
I asked someone to prove that ONE of the Church Fathers believed this.
I believe that was a reasonable application of a logical argument. I’ll state it slightly differently to show the manner to which it would apply here:
“Find one example where the early church fathers wrote that there were NOT little green men on a planet orbiting Alpha Centauri. You won’t find one, good luck. Therefore, it is clear that the early church fathers did NOT believe that there were not little green men on a planet orbiting Alpha Centauri”.
Your original argument, voiced in the positive, only shows that the early church fathers did not see fit to argue one way or another about whether Mary was emaculately conceived. It does not show that they didn’t believe, or that they did believe.
A positive argument would be to note the arguments of those who claimed the Bible taught the emaculate conception of Mary.