[Variant] Fix cast logic for Variant to Arrow for DataType::Null (#8825)

# Which issue does this PR close?

- Closes #8810 .


# What changes are included in this PR?

Fix the logic for `VariantToNullArrowRowBuilder` so that it respects the
`cast` option

# Are these changes tested?

added test

# Are there any user-facing changes?

Noe

Author: klion26

parquet-variant-compute/src/variant_get.rs

@@ -319,6 +319,7 @@ mod test {
     use arrow::compute::CastOptions;
     use arrow::datatypes::DataType::{Int16, Int32, Int64};
     use arrow::datatypes::i256;
+    use arrow::util::display::FormatOptions;
     use arrow_schema::DataType::{Boolean, Float32, Float64, Int8};
     use arrow_schema::{DataType, Field, FieldRef, Fields, IntervalUnit, TimeUnit};
     use chrono::DateTime;
@@ -1039,6 +1040,43 @@ mod test {
         arrow::array::NullArray::new(3)
     );
 
+    perfectly_shredded_variant_array_fn!(perfectly_shredded_null_variant_array_with_int, || {
+        Int32Array::from(vec![Some(32), Some(64), Some(48)])
+    });
+
+    // We append null values if type miss match happens in safe mode
+    perfectly_shredded_to_arrow_primitive_test!(
+        get_variant_perfectly_shredded_null_with_type_missmatch_in_safe_mode,
+        DataType::Null,
+        perfectly_shredded_null_variant_array_with_int,
+        arrow::array::NullArray::new(3)
+    );
+
+    // We'll return an error if type miss match happens in strict mode
+    #[test]
+    fn get_variant_perfectly_shredded_null_as_null_with_type_missmatch_in_strict_mode() {
+        let array = perfectly_shredded_null_variant_array_with_int();
+        let field = Field::new("typed_value", DataType::Null, true);
+        let options = GetOptions::new()
+            .with_as_type(Some(FieldRef::from(field)))
+            .with_cast_options(CastOptions {
+                safe: false,
+                format_options: FormatOptions::default(),
+            });
+
+        let result = variant_get(&array, options);
+
+        assert!(result.is_err());
+        let error_msg = format!("{}", result.unwrap_err());
+        assert!(
+            error_msg
+                .contains("Cast error: Failed to extract primitive of type Null from variant Int32(32) at path VariantPath([])"),
+            "Expected=[Cast error: Failed to extract primitive of type Null from variant Int32(32) at path VariantPath([])],\
+                Got error message=[{}]",
+            error_msg
+        );
+    }
+
     perfectly_shredded_variant_array_fn!(perfectly_shredded_decimal4_variant_array, || {
         Decimal32Array::from(vec![Some(12345), Some(23400), Some(-12342)])
             .with_precision_and_scale(5, 2)

parquet-variant-compute/src/variant_to_arrow.rs

@@ -696,24 +696,29 @@ impl VariantToBinaryVariantArrowRowBuilder {
     }
 }
 
-struct FakeNullBuilder(NullArray);
+#[derive(Default)]
+struct FakeNullBuilder {
+    item_count: usize,
+}
 
 impl FakeNullBuilder {
-    fn new(capacity: usize) -> Self {
-        Self(NullArray::new(capacity))
+    fn append_value(&mut self, _: ()) {
+        self.item_count += 1;
+    }
+
+    fn append_null(&mut self) {
+        self.item_count += 1;
     }
-    fn append_value<T>(&mut self, _: T) {}
-    fn append_null(&mut self) {}
 
     fn finish(self) -> NullArray {
-        self.0
+        NullArray::new(self.item_count)
     }
 }
 
 define_variant_to_primitive_builder!(
     struct VariantToNullArrowRowBuilder<'a>
-    |capacity| -> FakeNullBuilder { FakeNullBuilder::new(capacity) },
-    |_value|  Some(Variant::Null),
+    |_capacity| -> FakeNullBuilder { FakeNullBuilder::default() },
+    |value| value.as_null(),
     type_name: "Null"
 );