support nil checks on object fields

At present a variable's nil check is recognised as meaning the variable can't have a type of nil within the code block, but this doesn't work for object fields.

Setup code:

---@class MapPosition
---@field x integer
---@field y integer

---@class Data
---@field targetPosition MapPosition|nil

---@type Data
local data = {
    targetPosition = nil
}

---@param x MapPosition
local function demandMapPositionClass(x)
end

Object field code that doesn't work as it misses the nil check.

--- Object field nil check
local targetPos  ---@type MapPosition
if data.targetPosition ~= nil then
    targetPos = data.targetPosition -- This has set targetPos to a type of nil or MapPosition. But a nil value can't enter this block.
else
    targetPos = {x = 1, y = 1}
end
demandMapPositionClass(targetPos) -- This warns as targetPos can now contain nil.

Variable code that works correctly with it auto detecting the nil check.

--- Variable nil check
local targetPos2  ---@type MapPosition
local test = data.targetPosition
if test ~= nil then
    targetPos2 = test -- This correctly knows that test can only be of type MapPosition.
else
    targetPos2 = {x = 1, y = 1}
end
demandMapPositionClass(targetPos2)

I can fix the object field check with an @as on the offending line to force it to the right data type, but this isn't as futureproofed for later code changes. Also making a new variable just for this to magically works seems a waste and misleading when reading the code as you'd assume a variable is used later on for a specific purpose and not just to help Sumneko intellisense,

Inizia riproducendo il problema con la configurazione Lua e confronta il restringimento dei tipi per data.targetPosition con quello per la variabile di test locale. Traccia il percorso di analisi dei tipi che gestisce i controlli nil sui campi degli oggetti; il lavoro è completo quando l’esempio con il campo dell’oggetto non genera più un avviso quando targetPos viene passato a demandMapPositionClass.