fix(接口定义): 修复swagger导入case为空的问题

--bug=1009878 --user=刘瑞斌 【接口测试】接口定义，swagger导入报错 https://www.tapd.cn/55049933/s/1096270

backend/src/main/java/io/metersphere/api/dto/definition/parse/ApiDefinitionImport.java

@@ -7,6 +7,7 @@ import io.metersphere.base.domain.ApiTestCaseWithBLOBs;
 import io.metersphere.base.domain.EsbApiParamsWithBLOBs;
 import lombok.Data;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
@@ -17,7 +18,7 @@ public class ApiDefinitionImport {
     private List<ApiDefinitionWithBLOBs> data;
 
     // 新版本带用例导出
-    private List<ApiTestCaseWithBLOBs> cases;
+    private List<ApiTestCaseWithBLOBs> cases = new ArrayList<>();
 
     //ESB文件导入的附属数据类
     private Map<String,EsbApiParamsWithBLOBs> esbApiParamsMap;