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`.
Merge pull request #1450 from Elemecca/t-github-notify-error

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