Default class name missing when the spec attached to a parent without a suffix and prefix

### Intro

Hi!

I am Senior Software Engineer at Apptronik.

### My setup

Linux/MacOS, Mujoco C++, Mujoco 3.3.4

### What's happening? What did you expect?

When the child spec has a default class and it is attached to a parent class without using any suffix and prefix, the default class doesn't have any name. This results in an incompatible xml file.

### Steps for reproduction

1. Run the code below
2. Try loading the saved XML in the mujoco simulation application or using API.

### Minimal model for reproduction

This it the XML produced by my script

```XML

```

### Code required for reproduction

```
#include

using namespace std;

int main(){

mjSpec* spec = mj_makeSpec();
mjsBody* body = mjs_findBody(spec, "world");
mjsBody* body2 = mjs_addBody(body, nullptr);
mjsElement* frame = mjs_addFrame(mjs_findBody(spec, "world"), NULL)->element;

mjsDefault* def = mjs_addDefault(spec, "class", nullptr);

mjSpec* specA = mj_makeSpec();
mjsBody* bodyA = mjs_findBody(specA, "world");
mjsBody* bodyA2 = mjs_addBody(bodyA, nullptr);

mjsDefault* defA2 = mjs_addDefault(spec, "lol", nullptr);

mjs_attach(frame, bodyA2->element, "", "");

mj_compile(spec, nullptr);

char error[1024];
mj_saveXML(spec, "test.xml", error, sizeof(error));

return 0;
}
```

### Confirmations

- [x] I searched the [latest documentation](https://mujoco.readthedocs.io/en/latest/overview.html) thoroughly before posting.
- [x] I searched previous [Issues](https://github.com/google-deepmind/mujoco/issues) and [Discussions](https://github.com/google-deepmind/mujoco/discussions), I am certain this has not been raised before.