""与string.Empty是一样的，都表示空字符串，即这个string对象的值为空，但已经为其分配内存
null表示这个string对象为空，即这整个对象为空，没有为其分配内存
When a string variable is set to null, this means that it doesn't currently refer to a string instance.
An empty string is a string instance which contains no characters. In other words its Length property is 0.