#1974 tools/strata_bench.py: keep the ROCm driver read on the ROCm path

Description

On a machine with ROCm only (no NVIDIA tooling) `gpu_info_local()` fills `gpus` from
`rocm-smi --showproductname`'s Card Series lines. The list is therefore not empty, but the
`driver` field in it is `None`, so the `gpu_driver` field of the report picks that `None` and the
`rocm_driver` read a few lines above - in the same function - is thrown away. The header then
prints:

```
- OS: Linux 7.0.0-38-generic; GPU driver: not read; CUDA: not read
```

With the value used when it is present, it falls back to `rocm_driver`:

```
- OS: Linux 7.0.0-38-generic; GPU driver: 7.0.0-38-generic; CUDA: not read
```

Verified on a ROCm machine: `rocm-smi --showdriverversion` returns the version, the report said
"not read" before the change and shows the version after it. The affected field is header
metadata only; nothing in the measurement path changes, and `CUDA: not read` stays, which is
correct on an AMD-only box.