!425 优化默认值的问题，Model中不建议有默认值的逻辑

Merge pull request !425 from Yancey/default_value_optimization

@@ -25,7 +25,7 @@ public class LoginBody
     /**
      * 唯一标识
      */
-    private String uuid = "";
+    private String uuid;
 
     public String getUsername()
     {

@@ -100,7 +100,7 @@ public class SysRegisterService
      */
     public void validateCaptcha(String username, String code, String uuid)
     {
-        String verifyKey = Constants.CAPTCHA_CODE_KEY + uuid;
+        String verifyKey = Constants.CAPTCHA_CODE_KEY + StringUtils.nvl(uuid, "");
         String captcha = redisCache.getCacheObject(verifyKey);
         redisCache.deleteObject(verifyKey);
         if (captcha == null)