[JENKINS-75091] Fix null pointer exception when title is missing (#169) · jenkinsci/plot-plugin@da41f6b

"pull_request_target.opened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.reopened" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.synchronize" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

"pull_request_target.edited" is not a known webhook name (https://developer.github.com/v3/activity/events/types/)

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