Add --null-value flag to treat specific strings as NULL

[vmvarela]

Description

Treat a specific string (e.g., "N/A", "-", "NA") as NULL during loading.

Examples

$ cat data.csv
name,age
Alice,30
Bob,N/A
Carol,25

$ sql-pipe --null-value 'N/A' 'SELECT * FROM t WHERE age IS NOT NULL' data.csv
Alice,30
Carol,25

Acceptance Criteria

• --null-value <string> flag treats matching values as NULL during loading
• Works with all input formats
• Can be specified multiple times for multiple null sentinels
• Case-sensitive matching
• All existing tests pass
• New tests cover null value handling

Notes

• Implementation: ~20 lines
• Many real-world CSVs use sentinel values for missing data
• Currently these load as TEXT, breaking numeric aggregation
• High value for messy-data workflows