DRAFT PROPOSAL - Add option JSON_C_OBJECT_ADD_IF_NOT_NULL

This option of json_object_object_add_ex will cancel the
addition if the added value is NULL.

Remarks:
 - testing value with json_object_is_type(val, json_type_null)
   could make sense
 - that commit introduces a new test that could slow the speed
   of add

json_object.c

@@ -591,6 +591,9 @@ int json_object_object_add_ex(struct json_object *jso, const char *const key,
 
 	assert(json_object_get_type(jso) == json_type_object);
 
+	if (val == NULL && (opts & JSON_C_OBJECT_ADD_IF_NOT_NULL))
+		return 0;
+
 	// We lookup the entry and replace the value, rather than just deleting
 	// and re-adding it, so the existing key remains valid.
 	hash = lh_get_hash(JC_OBJECT(jso)->c_object, (const void *)key);

json_object.h

@@ -103,6 +103,11 @@ extern "C" {
  *       JSON_C_OBJECT_KEY_IS_CONSTANT);
  */
 #define JSON_C_OBJECT_KEY_IS_CONSTANT (1 << 2)
+/**
+ * A flag for the json_object_object_add_ex function which
+ * cancel the add if the value is NULL.
+ */
+#define JSON_C_OBJECT_ADD_IF_NOT_NULL (1 << 3)
 
 /**
  * Set the global value of an option, which will apply to all