Missing nullptr check in implicit std::string construction in RemotedHandler::unwrap

While experimenting with a replacement for XMLTooling's TemplateEngine, I ran into the following:

While rendering the session error template, if TemplateEngine->run() does not write any output to the ostream it is given, RemotedHandler::unwrap will implicitly construct a std::string from a nullptr, causing a basic_string::_S_construct NULL not valid exception to get thrown.

In the situation described above, h["data"].string() returns a nullptr, causing the implicit std::string constructor to throw an exception.

Not a huge issue since you need to go out of your way to make template rendering fail in ways that can trigger this – just a small robustness improvement. I've checked other similar (i/o)stringstream constructor sites to check for similar issues, but all others I've found have a check in place.