PQANALYSIS_LOG_FILE without PQANALYSIS_LOGGING_LEVEL crashes the import

```console
$ env -u PQANALYSIS_LOGGING_LEVEL PQANALYSIS_LOG_FILE=on python -c 'import PQAnalysis'
AttributeError: 'NoneType' object has no attribute 'lower'
```

`__init__.py:58` checks the off sentinel on the wrong variable (`logging_env_var` instead of `log_file_env_var`). The same typo means `PQANALYSIS_LOG_FILE=off` enables logging to a file literally named `off`.