crier/gh reporter: only create comment for valid PJs
crier/gh reporter: only create comment for valid PJs

The code contains several assumptions that the entering ProwJob slice
always contains non-nil `Refs` (which is not true for periodics). The
code filters the valid prowjobs but then uses the unfiltered slice,
leaving the not-to-be-reported Prowjobs in. This only happens in
summary_comment mode though: in default mode the slice always contains
exactly one prowjob and the existing safety checks are sufficient.

This is not a full fix for the case when someone deliberately enables GH
reporter for `periodics`, but it eliminates the problem for the default
case.

cjwagner/test-infra,cjwagner/test-infra,michelle192837/test-infra,michelle192837/test-infra,jessfraz/test-infra,jessfraz/test-infra,fejta/test-infra,cjwagner/test-infra,BenTheElder/test-infra,fejta/test-infra,fejta/test-infra,dims/test-infra,BenTheElder/test-infra,cblecker/test-infra,michelle192837/test-infra,BenTheElder/test-infra,monopole/test-infra,jessfraz/test-infra,BenTheElder/test-infra,monopole/test-infra,monopole/test-infra,cjwagner/test-infra,michelle192837/test-infra,monopole/test-infra,kubernetes/test-infra,monopole/test-infra,dims/test-infra,monopole/test-infra,fejta/test-infra,dims/test-infra,michelle192837/test-infra,cblecker/test-infra,dims/test-infra,fejta/test-infra,BenTheElder/test-infra,jessfraz/test-infra,kubernetes/test-infra,kubernetes/test-infra,cblecker/test-infra,cjwagner/test-infra,cblecker/test-infra,dims/test-infra,jessfraz/test-infra,dims/test-infra,michelle192837/test-infra,cblecker/test-infra,BenTheElder/test-infra,jessfraz/test-infra,cjwagner/test-infra,fejta/test-infra,cblecker/test-infra,kubernetes/test-infra,kubernetes/test-infra,kubernetes/test-infra