Add fields up to format version 700/13

Update StringField and IntegerField output methods to correctly handle
nil and empty values according to DATEV CSV format specifications.

Changes:
- StringField now returns empty string instead of quoted empty string
  for nil/empty values, ensuring reserved fields are completely empty
- IntegerField explicitly returns empty string for nil values
- Update tests to verify new nil/empty value handling
- Regenerate example CSV files with correct format

This resolves validation errors from DATEV Buchungsdatenservice API
where reserved fields must be completely empty (no quotes).

Fix CSV field formatting for DATEV API compliance