Fix incorrect null_count in generate_unions_case integration test (#…

…1713)

* Fix incorrect null_count

* Add check at reader too

arrow/src/ipc/reader.rs

@@ -239,9 +239,18 @@ fn create_array(
             Arc::new(array)
         }
         Null => {
-            let length = nodes[node_index].length() as usize;
+            let length = nodes[node_index].length();
+            let null_count = nodes[node_index].null_count();
+
+            if length != null_count {
+                return Err(ArrowError::IoError(format!(
+                    "Field {} of NullArray has unequal null_count {} and len {}",
+                    field, null_count, length
+                )));
+            }
+
             let data = ArrayData::builder(data_type.clone())
-                .len(length)
+                .len(length as usize)
                 .offset(0)
                 .build()
                 .unwrap();

arrow/src/ipc/writer.rs

@@ -860,7 +860,13 @@ fn write_array_data(
     null_count: usize,
 ) -> i64 {
     let mut offset = offset;
-    nodes.push(ipc::FieldNode::new(num_rows as i64, null_count as i64));
+    if !matches!(array_data.data_type(), DataType::Null) {
+        nodes.push(ipc::FieldNode::new(num_rows as i64, null_count as i64));
+    } else {
+        // NullArray's null_count equals to len, but the `null_count` passed in is from ArrayData
+        // where null_count is always 0.
+        nodes.push(ipc::FieldNode::new(num_rows as i64, num_rows as i64));
+    }
     // NullArray does not have any buffers, thus the null buffer is not generated
     // UnionArray does not have a validity buffer
     if !matches!(