Handle first waypoints with time_from_start == 0.

Two different scenarios have been addresses:

- Bugfix: If a motion containing exclusively no-planning joint is issued
  (e.g., center head), a reach time must be computed for the first waypoint.

- Cosmetic fix: The first motion point has zero time. Happens when equal to
  current position. We make the time_from_start be a small, almost zero
  value to prevent the controllers from issuing a warning.