While working on another PR, I noticed that the pipeline reports
a lot of results for testcase
'ci.pipeline.allpackages-build.package.ignored'. In all these
results, the 'original_spec_nvr' key is present, but its value
is an empty string. It looks to me like this code, when given
one of these messages, will try to publish a decision for a
package with the NVR "". I'm not sure how things go from there,
but I'm guessing either something later just bails on an empty
subject_identifier (best case) or it actually tries and comes
up with a nonsensical decision (medium case) or it causes some
kind of error or crash (worst case). Whichever of those it is,
it seems sensible to just avoid this by not yielding a subject
unless we actually have some content in the nvr field.

Here's an example of the fedmsg for one of these results:
https://apps.fedoraproject.org/datagrepper/id?id=2018-7d336210-10f9-4afa-88a2-2dcb8f32fb11&is_raw=true&size=extra-large

Signed-off-by: Adam Williamson awilliam@redhat.com