[json-core] Fix reading NULL value in a Map or List

blackbox-test/src/test/java/org/example/other/ObjectAdapterTest.java

@@ -30,6 +30,24 @@ void nullJson() {
     assertThat(asJson).isEqualTo("");
   }
 
+  @Test
+  void nullInMap() {
+    Object val = objectType.fromJson("{\"a\":1,\"b\":null,\"c\":3}");
+    assertThat(val).isInstanceOf(Map.class);
+
+    String asJson = objectType.toJson(null);
+    assertThat(asJson).isEqualTo("");
+  }
+
+  @Test
+  void nullInList() {
+    Object val = objectType.fromJson("[1,null,3]");
+    assertThat(val).isInstanceOf(List.class);
+
+    String asJson = objectType.toJson(val);
+    assertThat(asJson).isEqualTo("[1,3]");
+  }
+
   @Test
   void booleanTrue() {
     String asJson = objectType.toJson(true);

json-core/src/main/java/io/avaje/json/core/BasicObjectAdapter.java

@@ -60,6 +60,7 @@ public Object fromJson(JsonReader reader) {
       case BOOLEAN:
         return booleanAdapter.fromJson(reader);
       case NULL:
+        reader.isNullValue();
         return null;
       default:
         throw new IllegalStateException("Expected a value but was " + reader.currentToken() + " at path " + reader.location());

json-core/src/test/java/io/avaje/json/simple/SimpleMapperTest.java

@@ -32,6 +32,32 @@ void mapToJsonFromJson() {
     assertThat(mapFromJson2).isEqualTo(mapFromJson);
   }
 
+  @Test
+  void nullDirectly() {
+   var mapFromJson = simpleMapper.fromJson("null");
+    assertThat(mapFromJson).isNull();
+  }
+
+  @Test
+  void mapWithNull() {
+    Map<String, Object> mapFromJson = simpleMapper.fromJsonObject("{\"one\":1,\"two\":null,\"three\":3}");
+
+    assertThat(mapFromJson).containsKeys("one", "two", "three");
+    assertThat(mapFromJson.toString()).isEqualTo("{one=1, two=null, three=3}");
+
+    assertThat(simpleMapper.toJson(mapFromJson)).isEqualTo("{\"one\":1,\"three\":3}");
+  }
+
+  @Test
+  void listWithNull() {
+    List<Object> listFromJson = simpleMapper.fromJsonArray("[1,null,3]");
+
+    assertThat(listFromJson).hasSize(3);
+    assertThat(listFromJson.get(1)).isNull();
+
+    assertThat(simpleMapper.toJson(listFromJson)).isEqualTo("[1,3]");
+  }
+
   @Test
   void arrayToJsonFromJson() {