test: that empty filters list is serialized as 'null'

* this test currently fails

see #51

src/test/java/org/janelia/saalfeldlab/n5/zarr/N5ZarrTest.java

@@ -687,6 +687,7 @@ public void testReadZarrNestedPython() throws IOException, InterruptedException
 	@Test
 	public void testRawCompressorNullInZarray() throws IOException, ParseException, URISyntaxException {
 
+		// GsonBuilder's default value for serializeNulls is false
 		final ZarrKeyValueWriter n5 = (ZarrKeyValueWriter) createTempN5Writer();
 		n5.createDataset(
 				testZarrDatasetName,
@@ -704,6 +705,27 @@ public void testRawCompressorNullInZarray() throws IOException, ParseException,
 		}
 	}
 
+	@Test
+	public void testFiltersNullInZarray() throws IOException, ParseException, URISyntaxException {
+
+		// GsonBuilder's default value for serializeNulls is false
+		final ZarrKeyValueWriter n5 = (ZarrKeyValueWriter)createTempN5Writer();
+		n5.createDataset(
+				testZarrDatasetName,
+				new long[]{1, 2, 3},
+				new int[]{1, 2, 3},
+				DataType.UINT16,
+				new RawCompression());
+		final String zarrayLocation = n5.keyValueAccess.compose(n5.uri, testZarrDatasetName, ".zarray");
+		final LockedChannel zarrayChannel = n5.keyValueAccess.lockForReading(zarrayLocation);
+		final JSONParser jsonParser = new JSONParser();
+		try (Reader reader = zarrayChannel.newReader()) {
+			final JSONObject zarray = (JSONObject)jsonParser.parse(reader);
+			final JSONObject filters = (JSONObject)zarray.get("filters");
+			assertNull(filters);
+		}
+	}
+
 	@Test
 	@Override
 	public void testAttributes()  {