BE400A Captain reported an uncommanded nose gear extension in flight resulted in diversion to a successful landing.

Narrative

I Pilot In Command(PIC) was Pilot Flying; Second in Command (SIC) was Pilot Monitoring. We were in a descent from cruise altitude to an intermediate altitude. Around FL210; at an airspeed of 310 KIAS; we experience un-commanded NOSE gear deployment. The nose gear could be felt and heard to descend; and within moments indicated GREEN - down and locked. I immediately reduced power; deployed speed brakes; disengaged the autopilot; and raised the nose to reduce the rate of descent and reduce airspeed below 200 KIAS (VLO/VLE). Once the aircraft was safely under control; we verified the gear handle was in the UP position; the NOSE gear was down and locked; and that there was no applicable checklist. We contacted ATC; advised them of the situation and [requested priority]. We discussed possible courses of action and decided that landing at the destination was likely the best option. ATC informed us that our destination (ZZZ) did not have crash and fire rescue services. We discussed possible options and decided the safest course would be to divert to ZZZ1. We were unsure if the nose gear had taken damage during the un-commanded deployment and wanted rescue services available on landing. We informed ATC we would like to perform a low approach to the runway and requested tower verify the condition of our gear. On arrival; the main gear extended normally. We flew a normal visual approach and low approach over the runway. Tower informed us all gear appeared to be down and undamaged; however rescue services advised they thought the gear might not be all the way down. We performed a left traffic pattern and landed normally. No abnormal indications were observed after touchdown or on taxi.