Bail out of impact task if no regression range. (#2498)
Bail out of impact task if no regression range. (#2498)

* Bail out of impact task if no regression range.

The impact task is intended to abort if there is no regression range,
but it wasn't doing so. Instead we were returning an empty Impacts()
object which marked Security_Impact=Head but gave us no FoundIn label on
crbugs.

Bug: https://crbug.com/1222484
Change-Id: If94a9d812ebe6301e406145fa3a3927885ee096e

* Update impact_task.py

Co-authored-by: Oliver Chang <4872e2f9221282fc7b04cbd9bb5a261e7186a504@users.noreply.github.com>
google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz