file_mode and dir_mode do not correctly handle empty string case

If you do something like:

```
REPRO_ZIPFILE_DIR_MODE= python
```

Then you get

```
os.environ.get("REPRO_ZIPFILE_DIR_MODE", None)
#> ''
```

We should really be checking the truthiness of the value. Even if it were being entered as `0` or `000`, it would be a string and still be truthy.