Do atheists accept a null hypothesis when they say There is no God?
The simple answer is “no”. The ‘null hypothesis’ is a type of hypothesis used in statistics that proposes that no statistical significance exists in a set of given observations. The null hypothesis attempts to show that no variation exists between variables, or that a single variable is no different than zero. It is presumed to be true until statistical evidence nullifies it for an alternative hypothesis. Since God is in the realm of metaphysics, and no observations can be made, hypothesis testing cannot be applied, and therefore your question is illogical.