PropertyExtractor: Add null check for event.submitter

If the change was open, it would cause event.submitter to be
null. Fix this by adding a null check.

Gerrit's core does the same thing.

Bug: Issue 8051
Change-Id: I0690e184ba50ea1e60176e8be5af34c43a80393e
(cherry picked from commit d4659620aa109a9d9ea6641d18e9e473117cd22d)