`<empty>` listed in docs

[nicolasstucki]

http://dotty.epfl.ch/api/index.html is listing <empty> as a package. We should filter that one out.

[abgruszecki]

@romanowski Can we just skip packages that have no members? We probably want that anyway, and implementing that would fix this problem as well.

I'm not 100% certain if we don't ever want to display the magick <empty> package, since it can contain classes.

[nicolasstucki]

We should probably flatten the contents of <empty> into the root package.

[romanowski]

<empty> was introduced by dokka. The fix for now (that is also valid) is to remove all packages that does not have any members . See #10688

[abgruszecki]

Was fixed in #10688.