eina.Value.Empty now means that we have an zeroed (empty) eina value.
For optional values that are empty use eina.Value.OptionalEmpty.
This was required to support the empty values passed with
EINA_VALUE_EMPTY in some Ecore futures.
Also, returning an eina_value by value is not supported in eolian
for safety reasons, so we removed some tests that tried to use this
Depends on D6171