dev/core#1460 - Small cleanups in CiviEventDispatcher{,Test}

[totten]

Overview

This is a follow-up to the recently merged #17127 to address a couple small issues.

Before

• The unit test CiviEventDispatcherTest passes on some versions of PHP but raises a warning on others versions (This is because the GenericHootEvents aren't fully specified, leading to a call to count() on null value. See also: https://test.civicrm.org/job/CiviCRM-Core-Matrix/BKPROF=max,CIVIVER=master,SUITES=phpunit-civi,label=bknix-tmp/7995/testReport/Civi.Core/CiviEventDispatcherTest/testDispatchPolicy_run/)
• Some of the docblocks in CiviEventDispatcher have type-info but don't have "summary" lines.

After

• The unit test CiviEventDispatcherTest passes in more PHP environments.
• Some of the docblocks in CiviEventDispatcher have type-info but don't have "summary" lines.

[civibot]

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[colemanw]

Test failures are unrelated.