[XML] Specific behavior for self closing tags

[Marvin-Brouwer]

Is your feature request related to a problem? Please describe.
Currently an empty tag and a self closing tag behave the same.

Describe the solution you'd like
A self closing tag should relate to null and an empty tag should relate to empty by default.
However, this behavior should be configurable in the SerializerConfiguration.

Describe alternatives you've considered
-

Additional context
They used to both default to nullOrEmpty in some of the converters, this is not correct so it's changed.