@Nonnull
    public static Empty instance() {
        return INSTANCE;
    }
An object which represents nothing.

<p>Used mostly by {@link MetadataKey}s, where the presence of the key in the map
is enough for a behaviour to apply. In other words, the value is not significant.</p>

<p>Very similar to {@link Void}, except this class also provides an instance of the "empty" object.</p>
helper/src/main/java/me/lucko/helper/metadata/Empty.java