LoggingEvent#getFormattedMessage() doesn't format nulls the same way as SLF4J

[josephw]

LoggingEvent#getFormattedMessage() is public (#23). However, it doesn't present null values the same way as SLF4J, due to un-matched wrapping/unwrapping of Optional:

        LoggingEvent e = new LoggingEvent(Level.INFO, "Content: {}, {}", null, "value");
        System.out.println(e.getFormattedMessage());

Produces:

Content: Optional.empty, value

Rather than the expected:

Content: null, value

(Reposted from Mahoney#31)

[valfirst]

@josephw do you need a release for this fix?