～なくもない vs ～なくはない


> 自信がなく **も** ない
>
> 自信がなく **は** ない

What's the difference here? I understand that both constructions are basically
double negatives that cancel themselves into positives, with は sandwitched
there to emphasize the negative. But the question is, what is も's purpose
here, and what's the difference in meaning between the two constructions?

