Count nil values in the index

main.lua

box.cfg{}
statuses = { 'sent', 'delivered' }
uuid = require('uuid')
clock = require('clock')

box.schema.create_space('fragments')
box.space.fragments:create_index('pk')
box.space.fragments:create_index('status', { unique = false, parts = {{3, 'string', is_nullable = true}}})

for i =1,500000,1 do box.space.fragments:insert({i,uuid.str(), statuses[math.random(0,2)]}) end

box.space.fragments.index.status:count('sent') -- 166695
box.space.fragments.index.status:count('delivered') -- 166836
box.space.fragments.index.status:count({box.NULL}) -- 166469