Assign type missmatch nil not cast away

[Nexela]

nil is not cast away when checking with if

---@class Entity
---@field direction integer
local ent = {}

---@class Event
---@field start? integer
local event = {}

---@param event Event
local function f(event)
    if event.start then
        ent.direction = event.start
    end
end

[sumneko]

Only local variables support type tracking. At present, I have made special treatment for parameter checking, which will remove the nil of non-local variables. Assignment checks should do the same.