Stop dropping old deltas when there are no new deltas

Stop dropping old deltas when there are no new deltas

__vrps_update() wasn't properly differentiating between a validation
cycle that errored from one that generated no deltas. Both were being
handled as failure.

Therefore, Fort might have been inducing more RTR Cache Resets than it
was supposed to. Although, iterations in which no deltas are generated
seem to be rare.

The removal of surplus error propagation in the previous commit
revealed that compute_deltas() cannot actually fail, which made
the solution here trivial.