[Variant] Add VariantArrayBuilder::append_nulls API (#9685)

# Which issue does this PR close?

- Closes #9684.

# Rationale for this change

Check issue

# What changes are included in this PR?
- Add `VariantArrayBuilder::append_nulls`
- Add unit test
- add fast paths for `cast_to_variant_with_options` and
`string_array_to_variant`

# Are these changes tested?

Yes, added a unit test

# Are there any user-facing changes?
A new public API

Author: sdf-jkl

parquet-variant-compute/src/cast_to_variant.rs

@@ -58,6 +58,13 @@ pub fn cast_to_variant_with_options(
     input: &dyn Array,
     options: &CastOptions,
 ) -> Result<VariantArray, ArrowError> {
+    // Fast path: any all-null input maps to an all-null VariantArray.
+    if input.null_count() == input.len() {
+        let mut array_builder = VariantArrayBuilder::new(input.len());
+        array_builder.append_nulls(input.len());
+        return Ok(array_builder.build());
+    }
+
     // Create row builder for the input array type
     let mut row_builder = make_arrow_to_variant_row_builder(input.data_type(), input, options)?;
 

parquet-variant-compute/src/variant_array_builder.rs

@@ -156,6 +156,18 @@ impl VariantArrayBuilder {
         self.value_offsets.push(self.value_builder.offset());
     }
 
+    /// Appends `n` null rows to the builder.
+    pub fn append_nulls(&mut self, n: usize) {
+        self.nulls.append_n_nulls(n);
+        // The subfields are expected to be non-nullable according to the parquet variant spec.
+        let metadata_offset = self.metadata_builder.offset();
+        let value_offset = self.value_builder.offset();
+        self.metadata_offsets
+            .extend(std::iter::repeat_n(metadata_offset, n));
+        self.value_offsets
+            .extend(std::iter::repeat_n(value_offset, n));
+    }
+
     /// Append the [`Variant`] to the builder as the next row
     pub fn append_variant(&mut self, variant: Variant) {
         ValueBuilder::append_variant(self.parent_state(), variant);
@@ -526,6 +538,24 @@ mod test {
         assert_eq!(list.len(), 2);
     }
 
+    #[test]
+    fn test_variant_array_builder_append_nulls() {
+        let mut builder = VariantArrayBuilder::new(6);
+        builder.append_variant(Variant::from(1i32));
+        builder.append_nulls(0); // should be a no-op
+        builder.append_nulls(3);
+        builder.append_variant(Variant::from(2i32));
+
+        let variant_array = builder.build();
+
+        assert_eq!(variant_array.len(), 5);
+        assert_eq!(variant_array.value(0), Variant::from(1i32));
+        assert!(variant_array.is_null(1));
+        assert!(variant_array.is_null(2));
+        assert!(variant_array.is_null(3));
+        assert_eq!(variant_array.value(4), Variant::from(2i32));
+    }
+
     #[test]
     fn test_extend_variant_array_builder() {
         let mut b = VariantArrayBuilder::new(3);