fix(test): serializeNulls for consistency between Cached and non-cach…

…e versions

This is because when reading from the container with no cache, you get `null` but when writing/reading with the cache, the value is `JsonNull`.

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

@@ -778,7 +778,7 @@ public void testAttributes()  {
 	public void testAttributeMapping()  {
 
 		// attribute mapping on by default
-		try (final N5Writer n5 = createTempN5Writer()) {
+		try (final N5Writer n5 = createTempN5Writer(tempN5Location(), new GsonBuilder().serializeNulls())) {
 
 			n5.createDataset(datasetName, dimensions, blockSize, DataType.UINT64, getCompressions()[0]);