Do not call API if there is no API URL
Fixes #5878 (closed)
CE users may not be interested in the new JIRA features. In this case, we should detect they haven't set an API URL and fallback to the behavior pre-8.3. This patch does that very easily.
There are planned improvements to JIRA in future releases such as gitlab-org/gitlab-ce#5541 which will make this more configurable.