!343 上传图片时，选择的目录为空时，生成的图片访问不了

Merge pull request !343 from imalasong/pr/1

common-api/src/main/java/cn/lili/controller/common/UploadController.java

@@ -55,6 +55,9 @@ public class UploadController {
                                         String base64,
                                         @RequestHeader String accessToken, @RequestParam String directoryPath) {
 
+        if(StrUtil.isBlank(directoryPath)){
+            directoryPath = "default";
+        }
 
         AuthUser authUser = UserContext.getAuthUser(cache, accessToken);
         //如果用户未登录，则无法上传图片