Fix error in github-pull-request-notifier when body is empty.
Fix error in github-pull-request-notifier when body is empty.

When someone submits a pull request without a message the JSON
document contains `null` for `body`. The notifier script was
trying to call `match` on `body` to extract the list of user
mentions. That would fail when it was `null`, resulting in:

    Whoa, I got an error: TypeError: Cannot call method 'match' of null

This changes it to use the existential access operator, so
when body is `null` the mention list is `undefined`.

davidsulpy/hubot-scripts,azimman/hubot-scripts,arcaartem/hubot-scripts,dbkaplun/hubot-scripts,justinwoo/hubot-scripts,modulexcite/hubot-scripts,sklise/hubot-scripts,wsoula/hubot-scripts,jacobtomlinson/hubot-scripts,github/hubot-scripts,GrimDerp/hubot-scripts,jankowiakmaria/hubot-scripts,n0mer/hubot-scripts,MaxMEllon/hubot-scripts,yigitbey/hubot-scripts,alexhouse/hubot-scripts,zecahnin/hubot-scripts,chauffer/hubot-scripts,magicstone1412/hubot-scripts,phillipalexander/hubot-scripts,terryjbates/hubot-scripts,Ev1l/hubot-scripts,opentable/hubot-scripts,cycomachead/hubot-scripts,marksie531/hubot-scripts,ambikads/hubot-scripts,contolini/hubot-scripts,iilab/hubot-scripts,jan0sch/hubot-scripts