Difference between empty value and empty string for required attribute validation rule?

Could someone explain the difference between an empty value and an empty string. When I perform a validation check in a MF I always check whether a String = empty or String = ''. When is a string empty and when is a string ''? Also, how handles the validation rule required attribute on the domain model a string. Does it check both empty and '' or only empty?
1 answers

A "required" validation rule does indeed check for both empty and ''.

The difference between the two is a technical one: '' is a normal string value containing no characters, while empty (called null in most other systems) means it has no value at all. An Integer for example can also be empty, but cannot contain '' (as this is a string value, not a number).