get_job_logs silently returns empty iterator when no pod is found

When no pod is found for the requested step, the generator silently returns. The caller gets an empty iterator with no way to distinguish "job produced no logs" from "pod was never found."

```python
if pod_name is None:
return
```

| File affected | Line |
|--|--|
| `kubernetes/backend.py` | `line: 443` |
| `optimizer/backends/kubernetes/backend.py` | `line: 245` |

Same behavior appears in both files above

### Was this behavior intended or should it raise an error?

Read kubernetes/backend.py around line 443 and optimizer/backends/kubernetes/backend.py around line 245, starting at the pod_name check in get_job_logs. Confirm how both generators behave when no pod is found and resolve whether the missing pod should be distinguishable from an empty log result. Done means both implementations follow the decided behavior consistently.

Written by the indexing model from the issue text.