Plan should not be built if no factors defined

[dumoulin]

The ExplorationTask is responsible to build the attached plan using the defined factors. If no factors are defined, I think that the task should not try to built the plan. Indeed, most of implemented plan can implement test for being robuste to this case, but it will be more logical to assert that we have at least one factor defined when building the plan.

[romain.reuillon]

I think that a usable and a maintainable code should avoid singularities and have continuous behavior at the boundaries if possible. In this case it means that a plan with no factor should work. In any case it should lead to an empty built plan.