top_level startup code: fix infinite loop when defining a user path

[stanislaw]

Without this change, the code in the master branch goes into an infinite loop right at the start of any script that imports from ballcosmos.script import *.

The problem seems to be in how the join function works when it reaches the root path (/ on Linux).

I am only testing this change on Linux but I think it should be valid for Windows as well.

[ghost]

Thanks for the contribution. This whole code block is actually being removed in another change, so this pull request is unfortunately not need.