Correct websocket_test.cpp to accept NULL
Correct websocket_test.cpp to accept NULL

initialize a websocket without subprotocol is allowed, if the client don't ask
for a subprotocol or if the server don't agree about the requested protocols

gatzka/cjet,gatzka/cjet,gatzka/cjet,mloy/cjet,gatzka/cjet,mloy/cjet,gatzka/cjet,mloy/cjet,mloy/cjet,mloy/cjet