Do not send event if the buffer is empty.

Found many logs in bugreport that call sendEvents() even the buffer
is empty. We should avoid unnecessary API call to reduce ipc spam.
This change doesn't fix the potential ipc spam issue, only mitigates
the issue.

Bug: 231313913
Test: manual. Local add log and see the empty api call is skipped
Change-Id: I9c8afc5ae2d06074900886cf5743b1ec61ccb973