Advanced missing nil check

Nobody has claimed this yet.

This is probably really hard to "fix".
```lua
---@type table|nil
local blueprint = { x = 1 }
local hasBlueprint = true

if blueprint == nil then
hasBlueprint = false
end

if hasBlueprint then
error("no blueprint found")
end

local s = blueprint.x
```

Start by reproducing the provided Lua example and inspect the language server's nil-check and control-flow analysis. Confirm the expected diagnostic for the access to blueprint.x, then trace the relevant analyzer entry point and add or update focused coverage so the shown case is handled consistently.

Written by the indexing model from the issue text.