Not linked to any issue.

Currently when we want to create a new empty Entity, we need to transform entity_null_id to entt::entity and use it. It will force us to init entity like this:

ES::Engine::Entity new_entity(ES::Engine::Entity::ToEnttEntity(ES::Engine::Entity::entity_null_id))

Which is so fucking unconvenient :/

This PR allows to init entity like this:

ES::Engine::Entity new_entity;