[fix] Json parser must be able to parse null value

duniter4j-core-shared/src/main/java/org/duniter/core/util/json/JsonAttributeParser.java

@@ -123,6 +123,9 @@ public class JsonAttributeParser<T extends Object> {
 
         final String value = matcher.group(1);
         try {
+            if (value == null) {
+                return null;
+            }
             return parseValue(value);
         }
         catch(NumberFormatException | UnknownFormatConversionException e) {

duniter4j-core-shared/src/test/java/org/duniter/core/util/json/JsonAttributeParserTest.java

@@ -113,4 +113,32 @@ public class JsonAttributeParserTest {
         expectedJson = expectedJson.replace("\"ts\":\"" + TS_VALUE + "\",", "");
         Assert.assertEquals(expectedJson, newJson);
     }
+
+    @Test
+    public void parseNullableValue() {
+        // Long
+        {
+            String jsonString = "{\n" +
+                    "    \"ts\": null\n" +
+                    "}";
+
+            // nullable = true
+            JsonAttributeParser<Long> tsAttribute = new JsonAttributeParser<>(PROPERTY_TS, Long.class);
+            Long parsedValue = tsAttribute.getValue(jsonString);
+            Assert.assertNull(parsedValue);
+        }
+
+        // String
+        {
+            String jsonString = "{\n" +
+                    "    \"id\": null\n" +
+                    "}";
+
+            // nullable = true
+            JsonAttributeParser<String> idAttribute = new JsonAttributeParser<>(PROPERTY_ID, String.class);
+            String parsedValue = idAttribute.getValue(jsonString);
+            Assert.assertNull(parsedValue);
+
+        }
+    }
 }
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