本方法仅适用于 freemarker&springboot
IDEA控制台这样：Tip: If the failing expression is known to be legally refer to something that’s sometimes null or missing, either specify a default value like myOptionalVar!
错误又是：Caused by: freemarker.core.InvalidReferenceException: The following has evaluated to null or missing
原因：有变量是空值但是却${}了出来导致的
解决方法：在改变量后面补一个感叹号和一个字符串。
如果为空，就以默认值（“!”后的字符）显示。
举例：如果变量为：user，那么可以更改为${user!""}