"There is a god" is a claim.
"There is no god" is it's contrapositive.
The rules of logic apply to both claims. If you say that the lack of evidence against god is not proof for god, then you would be correct. But, you fail to consider that the lack of evidence for god does not equate to evidence against god.
Claims don't default to false in the absence of evidence. They start out as unknown.
Argument from ignorance - Wikipedia, the free encyclopedia