fix(themer-demo): Fix is_legacy_version

[gadenbuie]

Fixes #916 by correctly setting the is_legacy_version variable when the theme is version 3 or 4.

In short, theme_version(bs_global_get()) can return NULL, in which case we can fall back to version_default().

Actually, after fixing the above issue I realized that the themer demo didn't work correctly in this case anyway. Instead of falling back to the default version, we now fall back to the default theme, much earlier in the app.

theme <- bs_global_get() %||% bs_theme()