fix arrow-json encoding with dictionary including nulls (#6503)

* fix arrow-json encoding with dictionary including nulls

* linting

* fmt, and rename

arrow-json/src/writer/encoder.rs

@@ -409,7 +409,7 @@ impl<'a, K: ArrowDictionaryKeyType> DictionaryEncoder<'a, K> {
         array: &'a DictionaryArray<K>,
         options: &EncoderOptions,
     ) -> Result<Self, ArrowError> {
-        let encoder = make_encoder(array.values().as_ref(), options)?;
+        let (encoder, _) = make_encoder_impl(array.values().as_ref(), options)?;
 
         Ok(Self {
             keys: array.keys().values().clone(),

arrow-json/src/writer.rs → arrow-json/src/writer/mod.rs

@@ -111,8 +111,7 @@ use std::{fmt::Debug, io::Write};
 use arrow_array::*;
 use arrow_schema::*;
 
-use crate::writer::encoder::EncoderOptions;
-use encoder::make_encoder;
+use encoder::{make_encoder, EncoderOptions};
 
 /// This trait defines how to format a sequence of JSON objects to a
 /// byte stream.
@@ -1806,4 +1805,32 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn test_writer_null_dict() {
+        let keys = Int32Array::from_iter(vec![Some(0), None, Some(1)]);
+        let values = Arc::new(StringArray::from_iter(vec![Some("a"), None]));
+        let dict = DictionaryArray::new(keys, values);
+
+        let schema = SchemaRef::new(Schema::new(vec![Field::new(
+            "my_dict",
+            DataType::Dictionary(DataType::Int32.into(), DataType::Utf8.into()),
+            true,
+        )]));
+
+        let array = Arc::new(dict) as ArrayRef;
+        let batch = RecordBatch::try_new(schema, vec![array]).unwrap();
+
+        let mut json = Vec::new();
+        let write_builder = WriterBuilder::new().with_explicit_nulls(true);
+        let mut writer = write_builder.build::<_, JsonArray>(&mut json);
+        writer.write(&batch).unwrap();
+        writer.close().unwrap();
+
+        let json_str = str::from_utf8(&json).unwrap();
+        assert_eq!(
+            json_str,
+            r#"[{"my_dict":"a"},{"my_dict":null},{"my_dict":null}]"#
+        )
+    }
 }