My question is: when we talk about the null hypothesis here, are we referring to the sampling distribution constructed:
Before any data is collected (a priori), or
After data has been collected (a posteriori)?
In a one-sample t-test, for example, it is a combination.
A priori, you decide your null hypothesis, say $\mu=0$ (in general, $\mu = \mu_0$ for some constant $\mu_0$).
A posteriori, you calculate the sample variance, the sample mean, and the sample size.
All four components go into calculating the sampling distribution, test statistic, and p-value.
$$ \dfrac{\bar X - \mu_0}{s/\sqrt{n}} \sim t_{n - 1} $$
These terms a priori and a posteriori allude to and sound like, but are not the same as, the ideas of prior and posterior distributions in Bayesian statistics.