Remove void as possible option type (#7731)

Remove void as possible option type (#7731)

This PR removes the possibility to have void options. While there are (very) few options that may legitimately be considered void (they do not actually hold a value, but merely trigger a handler or predicate), the type being void introduces a number of special cases in the options setup. Their only real benefit was to avoid having data members that would never be used.
As it turns out, though, the same can be achieved by not specifying a name, which already supported as well.