Is it better to return NULL or empty values from functions/methods where the return value is not present?

Question

I am looking for a recommendation here. I am struggling with whether it is better to return NULL or an empty value from a method when the return value is not present or cannot be determined.

Take the following two methods as an examples:

string ReverseString(string stringToReverse) // takes a string and reverses it.
Person FindPerson(int personID)    // finds a Person with a matching personID.

In ReverseString(), I would say return an empty string because the return type is string, so the caller is expecting that. Also, this way, the caller would not have to check to see if a NULL was returned.

In FindPerson(), returning NULL seems like a better fit. Regardless of whether or not NULL or an empty Person Object (new Person()) is returned the caller is going to have to check to see if the Person Object is NULL or empty before doing anything to it (like calling UpdateName()). So why not just return NULL here and then the caller only has to check for NULL.

Does anyone else struggle with this? Any help or insight is appreciated.