fix(nullable): clear pointer destinations on null rows

[014-code]

Summary

• Clear pointer-to-pointer scan destinations when Nullable returns NULL, so reused dynamic scan variables no longer keep the previous decimal value.
• Add a unit test for Nullable clearing **decimal.Decimal.
• Add a regression test for Unexpected behavior when scanning Nullable(Decimal64(2)) column #1730 covering Nullable(Decimal64(2)) with repeated dynamic scanning.

Related issue

Closes #1730

Testing

• go test ./lib/column
• go test ./tests/issues -run Test1730NullableDecimalDynamicScan -count=1 -v (compiled, skipped on this host because Docker is unavailable)