setup.py: Do not error out when man directory is empty/missing

If the `man` directory is empty, then it won't be copied in the source
distribution, and `list_man_pages()` will throw an exception when trying
to list files in a non-existent directory. This prevents us from
installing the BuildStream package when the man pages are not there.

The most common use-case for this is when we want to re-generate the man
pages but want to install the package before re-generating them.