"Serialize the user prompt handler" algorithm should probably have a special case for WebDriverBiDi-only session

[lutien]

At the moment, running "serialize the user prompt handler" algorithm in case of BiDi when a client didn't specify unhandledPromptBehavior capability will resolve to dismiss and notify value which is not compatible with BiDi response for "session.new" command. So we probably should have a special case here if BiDi-only session was started.