`need-check-nil` when nil check is done in a function

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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

No type checking errors

### Actual Behaviour

`Need check nil.Lua Diagnostics.(need-check-nil)` on the `:lower()`

### Reproduction steps

Simplified case with the issue
```lua
---@param str string?
---@return boolean
local function isEmpty(str)
return str == nil or str == ''
end

---@param input string?
---@return string?
local function myFunc(input)
if isEmpty(input) then
return
end
return input:lower()
end
```

### Additional Notes

_No response_

### Log File

_No response_

Start by running the Lua example in the issue through the language server and inspect the type-checking path that produces the need-check-nil diagnostic. Trace how the isEmpty function result affects narrowing of input, then verify that the diagnostic is absent for input:lower() while nil checks elsewhere remain correct.

Written by the indexing model from the issue text.