Agnostics believe in nothing, in the sense that they do not believe in either the presence or the absence of deity - so they have an absence of belief in the presence of a deity (just as they have an abence of belief in the presence of deity). This is different from the atheist belief in nothing, which is the presence of a belief in the absence of a deity.

But both these stances are reasonably subsumed by the ‘nothing’ half of your ‘belief in something or nothing’ phrase, mainly because you have been imprecise in your definition of what the something or the nothing are in which each is purported to or to not believe in.