I haven't heard of that, but nsec is just a bech32 encoded (integer) number with checksum at the end. For instance nsec for the private key of 1 would be nsec1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqsmhltgl

Zero as a private key won't work cause it doesn't give you a valid point at the curve ie. a valid public key.