Draft: allways initialize callback argouts to empty list

This is one of the possible solutions to this pytango issue: pytango#590 (closed) I post it to discuss if this should be fixed directly in CppTango, or we should just fix it directly in PyTango boost layer.

Summary of the problem: device_proxy.read_attribute_asynch("attr", callback) was not working, failing with segfault, because ev->argout was a not initialized pointer 0x0. It was being passed from cpptango here to PyCallBackAutoDie::attr_read to PyDeviceAttribute::convert_to_python in PyTango boost. And finally this line tried to dereference it with ->empty() and it crashed.

The question to CppTango people - do you want to keep this pointer uninitialized for some reason in CppTango and I should just check this in PyTango boost layer and convert argout to None, or like in my MR you prefer to initialize it (then in PyTango it will be converted to an empty list)?