fix cast for ZSTD

Signed-off-by: Andrew Duffy <[email protected]>

Author: a10y

encodings/zstd/src/compute/cast.rs

@@ -10,9 +10,9 @@ use crate::{ZstdArray, ZstdVTable};
 
 impl CastKernel for ZstdVTable {
     fn cast(&self, array: &ZstdArray, dtype: &DType) -> VortexResult<Option<ArrayRef>> {
-        if !dtype.is_nullable() || !array.all_valid() {
-            // We cannot cast to non-nullable since the validity containing nulls is used to decode
-            // the ZSTD array, this would require rewriting tables.
+        if !dtype.is_nullable() && !array.all_valid() {
+            // If there are nulls, then the cast is not trivial and requires decoding
+            // to rewrite the tables.
             return Ok(None);
         }
         // ZstdArray is a general-purpose compression encoding using Zstandard compression.