simtime: use time=0 by default

This is the behaviour documented here:
http://wiki.ros.org/Clock#Using_Simulation_Time_from_the_.2BAC8-clock_Topic

    If the /use_sim_time parameter is set, the ROS Time API will return
    time=0 until it has received a value from the /clock topic. Then,
    the time will only be updated on receipt of a message from the
    /clock topic, and will stay constant between updates.

This resolves a NullPointerException when a node logs from its onStart
method and use_sim_time is true.