Add ability to skip JIRA report creation

We use the JIRA report in a reactor build, using Maven's reporting plugin inheritance to have child projects generate their own reports. However, some of the projects - such as the reactor build POMs themselves - have no real relevant JIRA components to be used in the report (they're just build aggregates). Right now, we just short-circuit the report in those cases by providing it a phony component ID, but it'd be nice if we could just skip it altogether, since our current approach still generates a JIRA report, but with an error message.

I've attached a patch with the proposed changes.