Null distribution, defined as the asymptotic distributions of null quantile-transformed test statistics  is in the procedure of hypothesis testing (testable on the basis of observed data)
The null distribution is defined as the asymptotic distributions of null quantile-transformed test statistics, based on marginal null distribution. In statistical hypothesis testing, the null distribution is the probability distribution of the test statistic when the null hypothesis is true. A statistical hypothesis is a hypothesis that is testable on the basis of observed data modeled as the realised values taken by a collection of random variables. 
