Can I get the original value if variable empty

Hi, I need to know if there is a variable in my text that is empty after expanding it. I need those variables not to expand and use the original string.

```golang
os.Setenv("GREETINGS", "hello")
value := "$GREETINGS $NAME"
result, err := envsubst.String(value)
if err != nil {
return "", fmt.Errorf("error expanding environment on '%s': %s", value, err.Error())
}
fmt.Println(result) // The output is: "hello "
```

Is there a way of getting `hello $NAME` as the output?

Look at the envsubst package's substitution logic, likely in the main source file. Understand how variables are expanded and where empty ones are handled. The goal is to modify the behavior so that undefined variables remain as their original string (e.g., $NAME) instead of being replaced with an empty string. Check existing tests to see how edge cases are handled and add a test for this new behavior.

索引モデルが issue の本文から書いたものです。