flow 条件表达式没有位置限制

```lua
---@type table<string, string>?
local shopUnit
local is_init = shopUnit == nil -- is_init 在前面赋值
shopUnit = {}
if is_init then
    A = shopUnit -- 此时 shopUnit 为 nil, 但他应该是 table<string, string>
end
B = shopUnit     -- 此时是 table?, 但应该是 table<string, string>
```