Remove branching when calling a wrapper method

[acki-m]

All wrappers (property, method, constructor, destructor & enumeration) currently hold a pointer to the wrapper base class.
When the client holds a valid item, the pointer is set, otherwise its a nullptr.
Every invoke of a member method for the wrappers mentioned above, checks for nullptr.
That's an unnecessary check, just provide for the invalid case a derived class,
which handles this case. All if checks could be removed.

To get the idea:

Current implementation:

variant method::invoke(instance object) const
{
    if (is_valid())
        return m_wrapper->invoke(object);
    else
        return variant();
}

New implementation:

variant method::invoke(instance object) const
{
    return m_wrapper->invoke(object);
}

m_wrapper would point to the a derived class which implement the invalid case.

[acki-m]

fixed with merging branch: rmv_if_branch_from_wrapper