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Раз вы не Раиса, то нам не о чем разговаривать.
... then we have nothing to talk about.
To convey the idea of "nothing", I usually expect to see the combination of "ни", "не", and "что", as in "ни на что не годится". Why the omission of "ни"?
Or perhaps, is it more related to the 4th definition of "не"?
Had it been used with a noun, it would be разговор ни о чём, however in this case it's не. And again, it should just be memorized. Here are some relevant articles from wiktionary: ни о чём, не о чем
So, не о чем indicates that there's nothing to talk about, while ни о чём sort indicates that you are talking about nothing [else].
For instance, in this example it will be ни as well:
Будем думать только об экзамене, и больше ни о чём.
Or consider following pair:
Больше ни о чём я вам не расскажу, ни в чём не признаюсь". (literally - I will tell you about nothing, I will confess nothing).
and
Не о чем мне вам рассказывать, не в чем мне признаваться". (literally - I have nothing to tell you about, I have nothing to confess).
Handling this a little differently: "не" is straightforward "no/not", but "ни" may indeed mean "nothing", like @shabunc rightly says, but also, depending on context, may mean "neither" or "nor" or sort of pointer to "anything" or "whatever".
So, "не то" is "not that [something which was expected/needed/etc.]".
But: "ни то, ни другое" is "neither this, nor that".
And: "чтобы [one word!] не делать" is "[in order] to do not", but "что бы [two words!] ни делать[, лишь бы не работать]" is "to do anything [but work]" or "whatever's one going to do [one's not going to do work]".
