The abscence of evidence for Jesus saying X at most would support the conclusion that we don't know if Jesus said it or not. Such lack of evidence is not positive evidence for the negative conclusion that Jesus didn't say it. This is a basic logical and historical point.
How can the public know that for fifty years skeptical psychologists have been trying their best to replicate classic psi experiments, and with notable unsuccess? It is this fact more than any other that has led to parapsychology's perpetual stagnation. Positive evidence keeps coming from a tiny group of enthusiasts, while negative evidence keeps coming from a much larger group of skeptics