The row builder for DataType::Null does not currently enforce strict casting. All values are blindly treated as null, no matter the casting mode.

The simplest fix would be to adjust the null builder's definition:

define_variant_to_primitive_builder!(
     struct VariantToNullArrowRowBuilder<'a>
     |capacity| -> FakeNullBuilder { FakeNullBuilder::new(capacity) },
-    |_value|  Some(Variant::Null),
+    |value|  value.as_null(),
     type_name: "Null"
 );

and change the fake row builder's append_value method to suit:

 impl FakeNullBuilder {
     fn new(capacity: usize) -> Self {
         Self(NullArray::new(capacity))
     }
-    fn append_value<T>(&mut self, _: T) {}
+    fn append_value(&mut self, _: ()) {}
     fn append_null(&mut self) {}

... but that might produce clippy warnings about passing unit type as a function argument. If so, we'd need to adjust the value conversion to produce Some dummy value instead, e.g. value.as_null().map(|_| 0) or matches!(value, Variant::Null).then_some(0)

Also, the fake null builder should probably track how many "values" were "appended" and either produce a NullArray of that length or blow up if the call count disagrees with the array's length. The former is probably more correct than the latter, since it matches all the other builders for whom "capacity" is only a hint.

Originally posted by @scovich in #8796 (comment)