Nil check for functions

還沒有人認領這個 Issue。

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

NeoVim

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking, Diagnostics/Syntax Checking

### Expected Behaviour

```lua
---@class Type1
---@field maybe_fun? fun(a: integer)
---@field absolutely_nil nil

---@type Type1
local type {
maybe_fun = nil
absolutely_nil = nil
}

type.maybe_fun(5) -- ERROR "maybe_fun" is nil, add nil check
type.absolutely_nil() -- ERROR "absolutely_nil" is nil
```

### Actual Behaviour

```lua
---@class Type1
---@field maybe_fun? fun(a: integer)
---@field absolutely_nil nil

---@type Type1
local type {
maybe_fun = nil
absolutely_nil = nil
}
-- No error messages, luals does not complain at all
type.maybe_fun(5)
type.absolutely_nil()
```

### Reproduction steps

1. Run example

### Additional Notes

_No response_

### Log File

_No response_

該 issue 未指定原始檔案或測試；首先在 Linux 上的 NeoVim 中重現 Lua 範例，並追蹤透過 nil 型別欄位進行呼叫時的型別檢查與診斷進入點。完成的標準是：診斷回報所示的兩種 nil 呼叫情況，包括可選函式和型別為 nil 的欄位。

由索引模型根據 Issue 內容生成。