fix: correctly produce Kafka JSON nulls for int fields (#5796)

This fixes what was likely a copy-paste error, where the Kafka JSON
producing logic checked for `NULL_SHORT` for int fields. None of the
other field types were mishandled. It's worth noting that the json
producer will omit null fields by default, and to explicitly output a
JSON null (`{..., "myIntField": null, ...}`), the `boolean outputNulls`
must be set to `true` (exposed as `output_nulls` in python).

Fixes #5701
Cherry-pick of #5702

extensions/kafka/src/main/java/io/deephaven/kafka/publish/JsonKeyOrValueSerializer.java

@@ -214,7 +214,7 @@ private JSONFieldProcessor makeIntFieldProcessor(final String fieldName, final C
  @Override
  void outputField(final int ii, final ObjectNode node, final IntChunk<Values> inputChunk) {
  final int raw = inputChunk.get(ii);
- if (raw == QueryConstants.NULL_SHORT) {
+ if (raw == QueryConstants.NULL_INT) {
  if (outputNulls) {
  node.putNull(childNodeFieldName);
  }