fix: swagger2引用的实体对象没定义会报错

--bug=1009407 --user=陈建星 [ github#8970]接口导入：swagger 2.0 json文件导入后，有些接口参数没有导入到MeterSphere系统里。 https://www.tapd.cn/55049933/s/10885833

backend/src/main/java/io/metersphere/api/dto/definition/parse/Swagger2Parser.java

@@ -100,6 +100,8 @@ public class Swagger2Parser extends SwaggerAbstractParser {
         Set<String> pathNames = paths.keySet();
 
         this.definitions = swagger.getDefinitions();
+        if (this.definitions == null)
+            this.definitions = new HashMap<>();
 
         List<ApiDefinitionWithBLOBs> results = new ArrayList<>();