bug_get_bugnote_stats() returns array|false, not NULL

In PHP 7.3 and earlier, dereferencing a non-array variable simply
returns NULL; starting with 7.4, a SYSTEM NOTICE is triggered.

Adjusting the code to make sure the stats have indeed been returned
(i.e. we got an array) instead of relying on a !== null check, which
in any case wrong since bug_get_bugnote_stats() returns false, not NULL.

Also update PHPDoc for bug_get_bugnote_stats() to reflect that.

Fixes 0026439