when no candidate to uprev

We've seen during the development of "uprev piglit in mesa" a corner case.

When an uprev is integrated into the target project, there may be no newer commits to the dependency project when the tool runs again.

Now, the tool produces a "void" uprev. As a result, the piglit images will be rebuilt, wasting resources and producing an uprev candidate that doesn't provide any value to the chain.

In this case, the ci-uprev tool should realize its candidate to uprev is the one already in use, so skip the next actions.