Nulls, empty strings, and blanks: CSV semantics in real pipelines

CSV cannot distinguish NULL from empty string without side metadata, both often appear as consecutive commas. Warehouses and ORMs apply different defaults on load.

Conventions

• Document whether "N/A", "null", or blanks mean missing.
• Trim whitespace on ingest when policy allows.
• Avoid mixing sentinels across feeds for the same column.