Add test for empty modification.

[romaninsh]

[DarkSide666]

Empty audit records are generated because they are pushed in beforeSave hook,
but if there was no actual changes in data model, then afterSave hook
is never called, so we can't delete them automatically :(

AND worst thing about this is that we can't add this condition (below)
because then in audit push() save() we can't reload record and all audit
system goes down :(

SO currently we will have these empty audit records and no way to get rid of them.

            $a->addCondition([
                ['descr', 'not', null],
                ['request_diff', 'not', null],
                ['reactive_diff', 'not', null],
            ]);