feat: add --null-value flag for custom NULL representation

[vmvarela]

Description

Add a --null-value flag that controls how SQL NULL values are rendered in CSV, TSV, and table output. JSON always uses native null regardless of the flag.

# Default: NULL renders as "NULL"
printf "name,score\nAlice,30\nBob,\n" | sql-pipe "SELECT * FROM t"
# → Alice,30
# → Bob,NULL

# Custom: replace with "N/A"
printf "name,score\nAlice,30\nBob,\n" | sql-pipe --null-value "N/A" "SELECT * FROM t"
# → Alice,30
# → Bob,N/A

# Empty string for downstream tools that treat blanks as NULL
printf "name,score\nAlice,30\nBob,\n" | sql-pipe --null-value "" "SELECT * FROM t"
# → Alice,30
# → Bob,

Changes

File	Change
src/args.zig	Parse --null-value <string> and --null-value=<string>
src/format.zig	Thread null_value through CSV/TSV/JSON/XML writers
src/main.zig	Pass null_value to table, markdown, and format writers
src/table.zig	Table output uses configured null_value
src/markdown.zig	Markdown output uses configured null_value
src/xml.zig	XML output uses configured null_value
build.zig	122-line integration tests for null-value scenarios
README.md	Custom NULL representation subsection + flags table entry
docs/sql-pipe.1.scd	Option description + example in man page

Testing

All integration and unit tests pass:

zig build test    # 120+ bash-based integration tests — exit 0
zig build unit-test  # XML unit tests — exit 0
ziglint src build.zig  # No new violations

Closes

Closes #170