Don't filter by app_id if it's null

http://manage.dimagi.com/default.asp?131005
@dannyroberts fyi.  System forms don't appear to actually have "_MISSING_APP_ID" as their app id - it's just null.  This made system forms not show up in the submit history report.  This might be in error, but anyways, it's moot to filter by app id in those cases.  There's separate domain filtering, so I'm not concerned about a data leak here.