The farmers sleep at night and work during the day, as habitual diurnals. Being naturally nocturnal doesn't mean your body shuts down against your will and you're unable to function during the day; certainly not if you're a sapient species capable of planning your future and making informed choices to guide your behaviour.
Presumably this species has physical adaptations appropriate for nocturnality, such as oversized eyes, a reliance on hearing, etc, so these farmers may need to take care to protect their eyes from daylight. Being habitually crepuscular could also be an option, operating around dawn and dusk.
Some aspects of agriculture simply can't be done under star light, or by relying on scent or hearing more than vision. Discolouration in fruits, vegetables or the plant itself won't be apparent; pollinators like bees will be active during the day (as the sun is a powerful navigation tool) and so husbandry will in part rely on their active period; even harvesting – being able to see and collect parts of plants – will be difficult if your visual acuity is based on movement more than colour or edge-detection.
