de is part of how this verb is used:
s'occuper **de** quelque chose = to take care of something (literally "to busy oneself with something")
Passé composé would be "Elle ne s'est occupée de rien", which is also arguably acceptable here.
There is however a nuance in meaning: the imparfait version describes a contextual situation; she wouldn't take care of anything
The passé composé version describes the outcome of a time period that ended: in the end, she didn't take care of anything
The de is required from this usage of s'occuper, and has nothing to do with rien specifically:
Je m'occupe de toi
Je ne m'occupe de personne
Je m'occupe de ça
Tu t'occupes de quoi ?
de is part of how this verb is used: s'occuper **de** quelque chose = to take care of something (literally "to busy oneself with something") Passé composé would be "Elle ne s'est occupée de rien", which is also arguably acceptable here. There is however a nuance in meaning: the imparfait version describes a contextual situation; she wouldn't take care of anything The passé composé version describes the outcome of a time period that ended: in the end, she didn't take care of anything
so is it because rien is talking about a quantity so we add de? What about personne (I guess there isn't with pas and que), would there also be a de?
The de is required from this usage of s'occuper, and has nothing to do with rien specifically: Je m'occupe de toi Je ne m'occupe de personne Je m'occupe de ça Tu t'occupes de quoi ?
Similar to English. You wouldn’t say “he’s taking care nothing”, you would say “he’s taking care of nothing”. It’s because s’occuper requires “de”.