support nil checks on object fields

Dieses Issue hat noch niemand übernommen.

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:
```lua
---@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.
```lua
--- 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.
```lua
--- 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,

Beginne damit, das Problem mit dem Lua-Setup zu reproduzieren, und vergleiche die Typverengung für data.targetPosition mit der Verengung für die lokale Testvariable. Verfolge den Pfad der Typanalyse, der nil-Prüfungen an Objektfeldern verarbeitet; fertig ist die Aufgabe, wenn das Beispiel mit dem Objektfeld keine Warnung mehr ausgibt, wenn targetPos an demandMapPositionClass übergeben wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.