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

還沒有人認領這個 Issue。

### 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_

將提供的 Lua 範例複製到 Lua 檔案中，並比較直接變數與巢狀欄位的診斷結果，以重現此問題。追蹤處理 ipairs 和 pairs 引數的型別檢查路徑，然後驗證 nullable 欄位是否會收到與直接變數相同的 param-type-mismatch 診斷。

由索引模型根據 Issue 內容生成。