Required jobs scale very very poorly with population. I wouldn't be surprised if the rate was something less than 1 additional person in the workforce for every 10 additional people in the population.
That would mean that countries with larger populations would have higher unemployment rates, but that is not the case.
It would mean that any poor country could simply become wealthy by allowing unlimited immigration...Most poor countries are nicer relative to some worse-off country, so certainly there would not be a shortage of immigrants wanting to migrate.
I think the null hypothesis has to be that there is no correlation between population size and employment.