@generic return ignores question mark

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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

### Expected Behaviour

Type of `y` in example below is `integer?`

### Actual Behaviour

Type of `y` in example below is `integer`

### Reproduction steps

```lua
---@generic T
---@param test T
---@return T?
function Func(test)
end

--- @return integer?
function Func2()
end

local x = 0
local y = Func(x)
local z = Func2()
```

### Additional Notes

`T | nil` works as a workaround.

### Log File

_No response_

Reproduce the Lua snippet in the Visual Studio Code extension and inspect the inferred types for y and z, comparing T? with the T | nil workaround. Done means the generic @return T? preserves nil so y is integer?, while the existing Func2 result remains integer?.

Written by the indexing model from the issue text.