DeviceImpl::real_ctor: Avoid null being part of the returned string
We currently have in the tests logs lines like
attr_polling 19:14:16.465972 DEBUG device.cpp:1125 In DeviceImpl::add_version_info(key = omniORB, value = 4.3.2) attr_polling 19:14:16.465980 DEBUG device.cpp:1125 In DeviceImpl::add_version_info(key = zmq, value = 4.3.4^@)
and that is because the zmq version string ends in a \0. We don't need to add that as std::ostringstream::str() returns a std::string.
Bug introduced in 355fd216 (Resolve "IDL6-version_info implementation", 2024-05-02).