Signed-off-by: Wenqi Li <[email protected]>

### Description
allow `config_file=None` when running `python -m monai.bundle run`,

this enables convenient quick runs via CLI, for example:
```bash
python -m monai.bundle run my_test --my_test="$print('hello')"
```

```bash
python -m monai.bundle run my_test --my_test="{_target_: LoadImage}"
```

output after this PR:
```
`config_file` not provided for 'monai.bundle run'.
2022-11-04 12:16:42,463 - INFO - --- input summary of monai.bundle.scripts.run ---
2022-11-04 12:16:42,463 - INFO - > runner_id: 'my_test'
2022-11-04 12:16:42,463 - INFO - > my_test: {'_target_': 'LoadImage'}
2022-11-04 12:16:42,463 - INFO - ---


<monai.transforms.io.array.LoadImage object at 0x7f80e2d05790>
```

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Wenqi Li <[email protected]>