Check if approvals array is null before iterating

When a CommentAddedEvent is received, the approvals supplier will return
null for the ApprovalAttribute[] if no approvals were contained in the
comment. Make sure to check if the array is null before iterating over
it.

Bug: Issue 12320
Change-Id: I624a186a1e17cbd02fe5eaa28cb60c594498e6b5