Fix 337 : Reader\Csv::inferSeparator() generates a notice when the file is empty

[slaur]

This is:

- [ x ] a bugfix
- [ ] a new feature

Checklist:

• [ x ] Changes are covered by unit tests
• [ x ] Code style is respected
• [ x ] Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• [ ] CHANGELOG.md contains a short summary of the change
• [ ] Documentation is updated as necessary

Why this change is needed?

When loading an empty file with Reader\Csv, a notice is emitted because the inferSeparator() function tries to access an undefined array index. This can lead to an exception depending on your configuration.
Note that to detect the error in the first place, I changed the config to make phpunit fail on notices.

[PowerKiKi]

Thanks ! Merged in 79d86ef