Fix: enrichment CSV loader pads missing fields as empty string, not null #383

Bug

read_csv_to_batch() in crates/logfwd-io/src/enrichment.rs pads short CSV rows with String::new(), making it impossible to distinguish NULL from empty string in SQL (IS NULL vs = ''). The Arrow schema already marks columns as nullable.

Location

crates/logfwd-io/src/enrichment.rs — search for String::new() in the CSV row padding logic.

Fix

Use Option<String> or build a nullable StringArray where missing fields are null, not empty strings.

Acceptance criteria

• Short CSV rows produce null values, not empty strings
• IS NULL works correctly in DataFusion queries on enrichment tables
• Add a test with short rows verifying null vs empty
• Tests pass (just test)
• Read AGENTS.md and CONTRIBUTING.md before starting