Fix spurious warning ("execution should start at current state")

Whenever `group.move()` is called, the following spurious warning was
always printed:

   Execution of motions should always start at the robot's current state. Ignoring the state supplied as start state in the motion planning request

This is what happened:

* `group.move()` is called
* in `MoveGroupImpl::constructGoal()`:
  - when `considered_start_state_` is empty, `goal.request.start_state.is_diff` remains `false`
* in `MoveGroupMoveAction::executeMoveCallback_PlanAndExecute()`:
  - `planning_scene::PlanningScene::isEmpty(request.start_state) == false`

This led to the warning being printed.
Merge pull request #497 from mintar/fix_nonempty_start_state

Fix spurious warning ("execution should start at current state")