fix: error when meta-refresh tag is missing

[mortenpi]

get_meta_redirect_url can return nothing, which wasn't handled. This should fix the error in https://github.com/JuliaComputing/MultiDocumenter.jl/issues/74#issuecomment-1769215417

Although I think it might just kick the can down the road, since versions.js will also probably be missing..

Also rearranges the tests a bit, so that we have more than one set of "fixtures".