Incorrect [Using undefined variable] error on unassigned variable declarations in autoconcat (non-strict mode)

When having variable declarations in a script where that variable might never be assigned to, then an incorrect `Using undefined variable: @var` runtime error is given if that declaration is inside an autoconcat.
Example code:
```
string @var;
msg();
```

Problem:
__autoconcat__() gets the value of declaration `string @var`, causing this runtime error.

从 __autoconcat__() 开始，报告指出它会在非严格模式下读取未赋值声明的值。使用 `string @var;` 后跟 `msg()` 重现该示例，并验证当该声明从未被赋值时，autoconcat 不再引发 `Using undefined variable: @var`。

由索引模型根据 Issue 内容生成。