Proposal consider vararg returns as forced optional

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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?

Diagnostics/Syntax Checking

### Expected Behaviour

I suggest consider **vararg returns** as **optional** (can be `nil`) because no one guarantees that they will be filled in. The same goes for arguments, despite the fact that their type can also be optional or nilable.

If we return at least something, but we don't know the quantity, then the first argument won't be vararg - `any, any ...`.

Thus after that we do not need to additionally specify that the vararg returns is optional.

```lua
---@param a string
function test(a) end

---@param ... string
---@return string ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --no warn, expected optinal cannot match

---@param ... string?
---@return string? ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --warn optinal cannot match

---@param ... string|nil
---@return string|nil ...
function f(...) end
local a, b, c = f() --no warn
local a, b, c = f("1")

test(a) --warn nil cannot match
```

### Actual Behaviour

Vararg arguments are optional, but vararg returns are not.


### Reproduction steps

1. Make vararg return of some existing type.
2. It is guaranteed to be infinite - not optional or nil.

### Additional Notes

_No response_

### Log File

_No response_

Commencez par le traitement des diagnostics et de l’inférence de types du Lua language server pour les arguments et les valeurs de retour vararg, en utilisant les étapes de reproduction et les exemples comme guide du comportement. Vérifiez que les valeurs de retour vararg sont traitées comme optional ou nilable lorsque cela est approprié, tandis que les appels fournis produisent les avertissements attendus et aucun avertissement, respectivement.

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