I created `StringBytes::GetExternalParts` as a helper to extract any external data (e.g. external string or buffer). So this and the following if statement could be done with just:
```c++
char* data = NULL;
size_t len;
bool is_extern = node::StringBytes::GetExternalParts(
    from, const_cast<const char**>(&data), &len);
if (is_extern)
  return data;
```