LP#1893463: Protect against null emails

LP#1893463: Protect against null emails

Unique indexes on nullable columns will allow multiple conceptually
unique rows if the nullable columns are, in fact, NULL because NULL does
not equal itself.  This commit uses COALESCE to make sure that the
nullable email column in the reporter.schedule table gets a value of the
empty string for the purposes of the unique index.  The upgrade script
now also takes this into account and ignores the email column.

Signed-off-by: Mike Rylander <mrylander@gmail.com>