fix(接口测试): 修复JSON-SCHEMA空类型参数处理问题

--bug=1033206 --user=赵勇 【接口测试】github#27698， 接口请求体开启json-schema后，设置参数为空，实际请求时非空，不符合预期 https://www.tapd.cn/55049933/s/1439488

Signed-off-by: fit2-zhao <yong.zhao@fit2cloud.com>

api-test/backend/src/main/java/io/metersphere/api/exec/generator/JSONSchemaBuilder.java

@@ -287,7 +287,7 @@ public class JSONSchemaBuilder {
             JSONArray jsonArray = (JSONArray) root.get(PropertyConstant.MS_OBJECT);
             List<String> list = new LinkedList<>();
             JSONSchemaParser.toJsonString(jsonArray, list);
-            return list.toString();
+            return JSON.toJSONString(list);
         }
         return JSON.toJSONString(JSON.toMap(root));
     }

api-test/backend/src/main/java/io/metersphere/api/exec/generator/JSONSchemaParser.java

@@ -238,7 +238,7 @@ public class JSONSchemaParser {
             JSONArray jsonArray = (JSONArray) root.get(PropertyConstant.MS_OBJECT);
             List<String> list = new LinkedList<>();
             toJsonString(jsonArray, list);
-            json = list.toString();
+            json = JSON.toJSONString(list);
         } else {
             json = JSON.toJSONString(JSON.toMap(root));
         }

framework/sdk-parent/sdk/src/main/java/io/metersphere/commons/utils/JSON.java

@@ -127,6 +127,8 @@ public class JSON {
                 value = toMap((JSONObject) value);
             } else if (value instanceof JSONArray) {
                 value = toList((JSONArray) value);
+            } else if (value == JSONObject.NULL) {
+                value = null;
             }
             map.put(key, value);
         }