Stop PHP notice on Find Mailings page

[braders]

Overview

Stop PHP notice on Find Mailings page.

Before

PHP notices, as shown in this screenshot:

After

The notices are gone. (unscheduled and archived are initially set to false, and overwritten with true as needed further down the file. This is a pattern we've repeated elsewhere).

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[colemanw]

Looks good, thanks @braders

[eileenmcnaughton]

test this please