When it comes to research or investigations, evidence is vitally important. That being said, to say "there is no evidence of X" no matter what the variable may be, is a logical fallacy because you cannot prove that there is no evidence of X.

This sort of thing happens all the time with governments and politicians. It's the old "what they don't know cannot hurt them" trick.

So, does destroying evidence make something invalid? Well, no. It does not. Does saying "there is no evidence" make something invalid? No, it does not.

Another example: There is no evidence of God, therefore God does not exist. <---- Both claims are negative claims. To claim that there is no evidence is a negative claim and to claim that God does not exist is a negative claim.

So, not only does one need to have evidence, they also need to prove that the evidence itself, does indeed exist without just saying "oh but there is plenty of evidence."