org.jboss.logmanager.handlers.AsyncHandler.AsyncTask should call flush when queue runs empty

When using the org.jboss.logmanager.handlers.AsyncHandler in combination with handlers that use autoFlush=false it happens that output stalls when queue runs empty.
A workaround is to set autoflush=true, however this introduces a flush always, even if there are more records in the queue, which render the flush useless.

An optimization is to invoke flush() on the handlers if the AsyncTask's queue ran empty - since then there is nothing else to do anyhow.