fixed handling null in AQL deserializer

Author: rashtao

src/main/java/com/arangodb/tinkerpop/gremlin/utils/AqlDeserializer.java

@@ -50,7 +50,9 @@ public Object deserialize(JsonNode node) {
     }
 
     private Object doDeserialize(JsonNode node) throws IOException {
-        if (isEdge(node)) {
+        if (node == null) {
+            return null;
+        } else if (isEdge(node)) {
             EdgeData data = mapper.readerFor(EdgeData.class).readValue(node);
             return new ArangoDBEdge(graph, data);
         } else if (isVertex(node)) {

src/test/java/com/arangodb/tinkerpop/gremlin/arangodb/simple/AqlTest.java

@@ -75,6 +75,14 @@ public void shouldReadSimpleValue() {
         assertThat(result).isEqualTo("hello");
     }
 
+    @Test
+    public void shouldReadNullValue() {
+        String result = graph().<String>aql(
+                "RETURN null"
+        ).next();
+        assertThat(result).isNull();
+    }
+
     @Test
     public void shouldReadVertex() {
         Vertex v = graph.addVertex();