`get_namespace` needs to handle `None` case

### Describe the Bug

Required for #597. In particular, `cumulative_trapezoid` has this case.

### To Reproduce

```python
import glass
...
```

### Expected Behaviour

_No response_

### Actual Behaviour

_No response_

### Version In Use

2025.2.dev50+gedea32d

### Additional Context

```markdown
- Python version:
- Operating system:
```