libcalamares: provide pybind11 constructor for GS

This constructor (of the proxy object) is used only by
test-code (see also comment in the Boost implementation)
but needs to be made explicit now.

src/libcalamares/python/Api.cpp

@@ -642,6 +642,7 @@ PYBIND11_MODULE( libcalamares, m )
         .def( "setprogress", &Calamares::Python::JobProxy::setprogress );
 
     py::class_< Calamares::Python::GlobalStorageProxy >( m, "GlobalStorage" )
+        .def( py::init( []( std::nullptr_t p ) { return new Calamares::Python::GlobalStorageProxy( nullptr ); } ) )
         .def( "contains", &Calamares::Python::GlobalStorageProxy::contains )
         .def( "count", &Calamares::Python::GlobalStorageProxy::count )
         .def( "insert", &Calamares::Python::GlobalStorageProxy::insert )