fix[pco]: fill_null cast sliced validity

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

Author: joseph-isaacs

encodings/pco/src/compute/cast.rs

@@ -24,7 +24,7 @@ impl CastKernel for PcoVTable {
                 // array.
                 array
                     .sliced_validity()
-                    .clone()
+                    
                     .cast_nullability(dtype.nullability(), array.len())?
             } else {
                 array