efl_mono: Add support for Eina.Error/Empty in eina.Value

efl_mono: Add support for Eina.Error/Empty in eina.Value

Summary:
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
behavior.
Depends on D6171

Reviewers: felipealmeida

Reviewed By: felipealmeida

Subscribers: cedric, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6172