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.