Add a default constructor for `libmexclass.proxy.Proxy`

[kevingurney]

When defining a client MATLAB class that wraps a instance of libmexclass.proxy.Proxy, it would be convenient to have an argument-less default constructor so that client code can make use of MATLAB property validation:

classdef Client
    properties
        Proxy(1,1) libmexclass.proxy.Proxy
    end
end