Just show the coverage of a pipeline if the pipeline has it

It's mentioned in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7447#note_18724084

For now we're showing the coverage only if:

pipeline.project.build_coverage_enabled?

However since we introduced job coverage setting in .gitlab-ci.yml from !7447 (merged), pipeline could have coverage even if the project doesn't enable (set) it.

So we should just check if the pipeline has a corresponding coverage, otherwise it would be a gotcha that people also need to set coverage setting in the project in order to see the pipeline coverage.

Thanks @leandroico for reminding us this!

/cc @grzesiek