handling args.json has compile errors

[stephanschulz]

I am on macOS 10.14.6 with OF 0.11 trying to run the example_client_sharedcanvas because I wanted to learn how to parse the args in 'onMessage() but get json related compile errors here: https://github.com/robotconscience/ofxLibwebsockets/blob/3d4843a840d73270fee7e812ba7983697c523cbd/example_client_sharedcanvas/src/ofApp.cpp#L90

/Applications/of_v0.11.0_osx_release/addons/ofxLibwebsockets/example_client_sharedcanvas/src/ofApp.cpp:95:33: No member named 'isNull' in 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator, adl_serializer>'; did you mean 'is_null'?