Fix semantic conflict with array slots

Signed-off-by: Robert Kruszewski <github@robertk.io>

Author: robert3005

vortex-array/src/arrays/masked/execute.rs

@@ -224,9 +224,7 @@ fn mask_validity_variant(
             // Child has an array-backed validity stored as its first child.
             // Combine with the mask and replace that child via with_children.
             let combined = combine_validity(&child_validity, mask, len, ctx)?;
-            let mut children = child.children();
-            children[0] = combined.to_array(len);
-            let new_child = child.with_children(children)?;
+            let new_child = child.with_slot(0, combined.to_array(len))?;
             Ok(VariantArray::new(new_child))
         }
     }