Fix warning with mrtydi

Warning is probably much more pervasive, but came across it running the following:

```
python -m pyserini.2cr.mrtydi --condition mdpr-tied-pft-msmarco --language ru --display-commands --directory runs/
```

The warning:

```
Asking to truncate to max_length but no maximum length is provided and the model has no predefined maximum length. Default to no truncation.
```