support nil checks on object fields

Nobody has claimed this yet.

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,

Start by reproducing the issue with the Lua setup and compare type narrowing for data.targetPosition with narrowing for the local test variable. Trace the type-analysis path that handles nil checks on object fields; done means the object-field example no longer warns when targetPos is passed to demandMapPositionClass.

Written by the indexing model from the issue text.