fix: tolerate null source import checksums

[danshapiro]

What changed

• Scan source import item checksums through sql.NullString in GetSourceImportItem and ListImportedSourceItemChecksums.
• Surface an empty checksum string when legacy/imported rows contain NULL.
• Add focused store coverage for NULL checksum rows.

Why

Some source_import_items rows can have NULL checksum values, which previously caused scan errors when imported source items were read back.

Impact

Nullable checksums no longer break source import lookup or imported checksum listing; non-NULL checksum behavior is unchanged.

Validation

• gofmt on touched Go files
• go test -tags "fts5 sqlite_vec" ./internal/store
• make testify-helper-check
• go vet -tags "fts5 sqlite_vec" ./...

[roborev-ci]

roborev: Combined Review (a9c82e7)

Verdict: No Medium, High, or Critical issues found.

The only reported finding was Low severity, so it is omitted per the review-combination rules.

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Panel: ci_default_security | Synthesis: codex, 5s | Members: codex_default (codex/default, done, 1m51s), codex_security (codex/security, done, 12s) | Total: 2m8s