Empty fields is error prone because it generates empty line.
formatter_csv: Change fields parameter to required
IMO it should raise error for empty field configured.
It seems good > raise when fields is empty.
raise ConfigError when fields are empty
Reduce memory allocation in each format call
Review applied. After test passed, will merge.
Please send another pull-req to master branch right now to raise error for empty fields.
It may make regressions if missed now.