fix(arrow/scalar): allow null extension scalars to keep null storage

arrow/scalar/scalar.go

@@ -421,10 +421,16 @@ func (e *Extension) Validate() (err error) {
 	}
 
 	if !e.Valid {
-		if e.Value != nil {
-			err = fmt.Errorf("null %s scalar has storage value", e.Type)
+		if e.Value == nil {
+			return nil
 		}
-		return
+		if e.Value.IsValid() {
+			return fmt.Errorf("null %s scalar has non-null storage value", e.Type)
+		}
+		if err = e.Value.Validate(); err != nil {
+			return fmt.Errorf("%s scalar fails validation for storage value: %w", e.Type, err)
+		}
+		return nil
 	}
 
 	switch {

arrow/scalar/scalar_test.go

@@ -33,6 +33,7 @@ import (
 	"github.com/apache/arrow-go/v18/arrow/decimal256"
 	"github.com/apache/arrow-go/v18/arrow/memory"
 	"github.com/apache/arrow-go/v18/arrow/scalar"
+	"github.com/apache/arrow-go/v18/internal/types"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -92,6 +93,23 @@ func checkMakeNullScalar(t *testing.T, dt arrow.DataType) scalar.Scalar {
 	return s
 }
 
+func TestMakeNullExtensionScalar(t *testing.T) {
+	dt := types.NewSmallintType()
+	sc := checkMakeNullScalar(t, dt)
+	ext := sc.(*scalar.Extension)
+	require.NotNil(t, ext.Value)
+	assert.False(t, ext.Value.IsValid())
+	assert.True(t, arrow.TypeEqual(dt.StorageType(), ext.Value.DataType()))
+}
+
+func TestNullExtensionScalarValidateRejectsNonNullStorage(t *testing.T) {
+	sc := scalar.NewExtensionScalar(scalar.NewInt16Scalar(1), types.NewSmallintType())
+	sc.Valid = false
+
+	assert.ErrorContains(t, sc.Validate(), "non-null storage value")
+	assert.ErrorContains(t, sc.ValidateFull(), "non-null storage value")
+}
+
 func TestMakeScalarUint(t *testing.T) {
 	three := scalar.MakeScalar(uint(3))
 	assert.NoError(t, three.ValidateFull())