Missing diagnostic when passing nil-able fields to pairs/ipairs

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### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Type Checking, Diagnostics/Syntax Checking

### Expected Behaviour

In the following code, I expect to get param-type mismatch for each of the `for` loops (with `pairs` and `ipairs`) because the variable being iterated can be `nil`:

```lua
---@type string[]?
local x1 = {}

for _ in ipairs(x1) do -- param-type-mismatch: Cannot assign `string[]?` to parameter `
break
end

---@type table?
local y1 = {}

for _ in ipairs(y1) do -- param-type-mismatch: Cannot assign `table?` to parameter `
break
end

---@class Class
---@field x string[]?
---@field y table?

---@type Class
local x2 = {}

for _ in ipairs(x2.x) do -- Should warn, but it doesn't
break
end

for _ in pairs(x2.y) do -- Should warn, but it doesn't
break
end
```

### Actual Behaviour

The param-type-mismatch diagnostic is only shown for the first two loops (direct variables without nesting/fields), but not when using fields (the third and fourth for loops).

### Reproduction steps

Copy the code into a Lua file and observe the diagnostics

### Additional Notes

_No response_

### Log File

_No response_

Reproduisez le problème en copiant l’exemple Lua fourni dans un fichier Lua et en comparant les diagnostics pour les variables directes et les champs imbriqués. Suivez le chemin de vérification des types qui gère les arguments de ipairs et pairs, puis vérifiez que les champs nullable reçoivent le même diagnostic param-type-mismatch que les variables directes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.