applier: do not pass success event to ballot update trigger

Ballot watcher used to pass a boolean as an event to ballot_update
triggers, with "false" meaning there was some error while waiting for
ballot and one has to check diag for the error.

This was intended as a guard against stale diag contents (diag might
contain some leftovers from previous errors).

Let's make ballot watcher fiber clear the diagnostics when it's not
planning on using them. Now ballot update triggers may rely on
`diag_is_empty()` test instead of the "success" parameter.

In-scope-of #8509

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring