chore[array]: fsl append null scalar (#5156)

Signed-off-by: Joe Isaacs <[email protected]>

Author: joseph-isaacs

vortex-array/src/builders/fixed_size_list.rs

@@ -71,6 +71,12 @@ impl FixedSizeListBuilder {
     ///
     /// [`ListArray`]: crate::arrays::ListArray
     pub fn append_value(&mut self, value: ListScalar) -> VortexResult<()> {
+        let Some(elements) = value.elements() else {
+            // If `elements` is `None`, then the `value` is a null value.
+            self.append_null();
+            return Ok(());
+        };
+
         if value.len() != self.list_size() as usize {
             vortex_bail!(
                 "Tried to append a `ListScalar` with length {} to a `FixedSizeListScalar` \
@@ -80,12 +86,6 @@ impl FixedSizeListBuilder {
             );
         }
 
-        let Some(elements) = value.elements() else {
-            // If `elements` is `None`, then the `value` is a null value.
-            self.append_null();
-            return Ok(());
-        };
-
         for scalar in elements {
             // TODO(connor): This is slow, we should be able to append multiple values at once, or
             // the list scalar should hold an Array
@@ -492,6 +492,28 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_append_scalar_nulls() {
+        // Elements must be nullable if we're going to append null lists
+        let dtype: Arc<DType> = Arc::new(DType::Primitive(I32, Nullable));
+        let mut builder = FixedSizeListBuilder::with_capacity(dtype, 2, Nullable, 0);
+
+        assert_eq!(builder.dtype().nullability(), Nullable);
+        builder
+            .append_scalar(&Scalar::null(builder.dtype().clone()))
+            .unwrap();
+        assert_eq!(builder.len(), 1);
+
+        let fsl = builder.finish();
+        assert_eq!(fsl.len(), 1);
+
+        let fsl_array = fsl.to_fixed_size_list();
+        assert_eq!(fsl_array.list_size(), 2);
+
+        // Check that all lists are null.
+        assert!(!fsl_array.validity().is_valid(0));
+    }
+
     #[test]
     fn test_append_zeros_degenerate() {
         let dtype: Arc<DType> = Arc::new(I32.into());