JENKINS-33166 check for nonnull url inside constructor

[escoem]

If you try to add and empty jira plugin configuration at Manage Jenkins, will fail with NPE.

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This change is

[rantoniuk]

Actually we shouldn't allow adding a JIRA site with an empty URL, what's the point ?:)
I think it is better to make the URL field required on the UI if possible.

[coveralls]

Coverage remained the same at 48.058% when pulling b7d807c on escoem:JENKINS-33166 into 138d215 on jenkinsci:master.