[REF] Fix pointless use of CRM_Utils_Array::value in Campaign BAO

[colemanw]

Overview

Just a small code cleanup

Before

!(CRM_Utils_Array::value('bar', $foo))

After

empty($foo['bar'])

[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).