Prior to commit v3.4.0-rc1~264^2~1 (GetPrerequisites: Add error checks
for execute_process() calls, 2015-07-29), `get_prerequisites` would
simply warn on a missing binary and not update the result list at all.
That commit accidentally made the case an error. This was fixed by
commit v3.8.0-rc1~110^2 (GetPrerequisites: Do not fail on files we
cannot find, 2017-01-10), but the fix also cleared the result list.
Clearing the list is incorrect because it is supposed to be able to
accumulate results over multiple calls.
Remove the list clearing behavior to restore the original behavior on a
missing binary.
Fixes: #17306
