Handle NullNode for optional attributes in Jackson CloudEventDeserializer (#432)

In `getOptionalStringNode` we should handle `JsonNode`s that are
instances of `NullNode`.

Signed-off-by: Pierangelo Di Pilato <pdipilat@redhat.com>

Author: pierDipi

formats/json-jackson/src/main/java/io/cloudevents/jackson/CloudEventDeserializer.java

@@ -24,6 +24,7 @@
 import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
 import com.fasterxml.jackson.databind.exc.MismatchedInputException;
 import com.fasterxml.jackson.databind.node.JsonNodeType;
+import com.fasterxml.jackson.databind.node.NullNode;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import io.cloudevents.CloudEvent;
 import io.cloudevents.CloudEventData;
@@ -203,12 +204,12 @@ private String getStringNode(ObjectNode objNode, JsonParser p, String attributeN
         }
 
         private String getOptionalStringNode(ObjectNode objNode, JsonParser p, String attributeName) throws JsonProcessingException {
-            JsonNode unparsedSpecVersion = objNode.remove(attributeName);
-            if (unparsedSpecVersion == null) {
+            JsonNode unparsedAttribute = objNode.remove(attributeName);
+            if (unparsedAttribute == null || unparsedAttribute instanceof NullNode) {
                 return null;
             }
-            assertNodeType(unparsedSpecVersion, JsonNodeType.STRING, attributeName, null);
-            return unparsedSpecVersion.asText();
+            assertNodeType(unparsedAttribute, JsonNodeType.STRING, attributeName, null);
+            return unparsedAttribute.asText();
         }
 
         private void assertNodeType(JsonNode node, JsonNodeType type, String attributeName, String desc) throws JsonProcessingException {

formats/json-jackson/src/test/java/io/cloudevents/jackson/JsonFormatTest.java

@@ -45,7 +45,7 @@
 
 class JsonFormatTest {
 
-    private ObjectMapper mapper = new ObjectMapper();
+    private final ObjectMapper mapper = new ObjectMapper();
 
     @ParameterizedTest
     @MethodSource("serializeTestArgumentsDefault")
@@ -198,6 +198,7 @@ public static Stream<Arguments> serializeTestArgumentsBase64() {
     public static Stream<Arguments> deserializeTestArguments() {
         return Stream.of(
             Arguments.of("v03/min.json", V03_MIN),
+            Arguments.of("v03/min_subject_null.json", V03_MIN),
             Arguments.of("v03/json_data.json", normalizeToJsonValueIfNeeded(V03_WITH_JSON_DATA)),
             Arguments.of("v03/json_data_with_ext.json", normalizeToJsonValueIfNeeded(V03_WITH_JSON_DATA_WITH_EXT)),
             Arguments.of("v03/base64_json_data.json", V03_WITH_JSON_DATA),
@@ -207,6 +208,7 @@ public static Stream<Arguments> deserializeTestArguments() {
             Arguments.of("v03/text_data.json", V03_WITH_TEXT_DATA),
             Arguments.of("v03/base64_text_data.json", V03_WITH_TEXT_DATA),
             Arguments.of("v1/min.json", V1_MIN),
+            Arguments.of("v1/min_subject_null.json", V1_MIN),
             Arguments.of("v1/json_data.json", normalizeToJsonValueIfNeeded(V1_WITH_JSON_DATA)),
             Arguments.of("v1/json_data_with_ext.json", normalizeToJsonValueIfNeeded(V1_WITH_JSON_DATA_WITH_EXT)),
             Arguments.of("v1/base64_json_data.json", V1_WITH_JSON_DATA),

formats/json-jackson/src/test/resources/v03/min_subject_null.json

@@ -0,0 +1,7 @@
+{
+    "specversion": "0.3",
+    "id": "1",
+    "type": "mock.test",
+    "source": "http://localhost/source",
+    "subject": null
+}

formats/json-jackson/src/test/resources/v1/min_subject_null.json

@@ -0,0 +1,7 @@
+{
+    "specversion": "1.0",
+    "id": "1",
+    "type": "mock.test",
+    "source": "http://localhost/source",
+    "subject": null
+}