[json] regression test for null value

A JSON null value is being translated to a “null” string.

Was supposed to be fixed in FasterXML/jackson-databind#1842 but it doesn’t appear to be.

modules/json/pom.xml

@@ -23,5 +23,13 @@
       <groupId>com.spotify.ffwd</groupId>
       <artifactId>ffwd-api</artifactId>
     </dependency>
+
+    <!-- testing -->
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
+
 </project>

modules/json/src/main/java/com/spotify/ffwd/json/JsonObjectMapperDecoder.java

@@ -169,7 +169,7 @@ private double decodeDouble(JsonNode tree, String name) {
         return n.asDouble();
     }
 
-    private String decodeString(JsonNode tree, String name) {
+    String decodeString(JsonNode tree, String name) {
         final JsonNode n = tree.get(name);
 
         if (n == null) {

modules/json/src/test/java/com/spotify/ffwd/json/JsonObjectMapperDecoderTest.java

@@ -0,0 +1,29 @@
+package com.spotify.ffwd.json;
+
+import static junit.framework.TestCase.assertNull;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import java.io.IOException;
+import org.junit.Before;
+import org.junit.Test;
+
+public class JsonObjectMapperDecoderTest {
+  private final ObjectMapper mapper = new ObjectMapper();
+
+  private JsonObjectMapperDecoder j;
+
+  @Before
+  public void setup() {
+   j = new JsonObjectMapperDecoder();
+  }
+
+  @Test
+  public void nullKey() throws IOException {
+    final JsonNode json = mapper.readTree("{\"key\": null}");
+    final String key = j.decodeString(json, "key");
+
+    assertNull(key);
+  }
+
+}