Nil check for functions

Nobody has claimed this yet.

### 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_

The issue names no source file or test; start by reproducing the Lua example in NeoVim on Linux and trace the type-checking and diagnostic entry point for calls through nil-typed fields. Done means diagnostics report both nil call cases shown, including the optional function and the field typed as nil.

Written by the indexing model from the issue text.