修复账户密码为空时，可以登陆的bug

src/main/java/com/github/zxbu/webdavteambition/config/EmbeddedTomcatConfig.java

@@ -37,7 +37,7 @@ public class EmbeddedTomcatConfig implements WebServerFactoryCustomizer<Configur
                     if (aliYunDriveProperties.getAuth().getUserName().equals(username)) {
                         return aliYunDriveProperties.getAuth().getPassword();
                     }
-                    return "";
+                    return null;
                 }
 
                 @Override