fix issues loading CSS in default slots (#61428)
### What
When using a default slot that re-exports another page, the CSS entries
(and presumably other client reference modules, like JS entries) that
correspond with the re-exported page won't be loaded, resulting in CSS
styles not being applied.
### Why
`next-app-loader` constructs the path to the default segment but special
cases the `children` slot (which is used as the page), to mark the
segment as an empty string. This is so that the parallel default file
can be loaded. However the way that `defaultPath` is constructed, it
assumes that `actualSegment` won't be an empty string, and constructs an
invalid path like so:
`app/example//default.tsx`
When we go to look up the JS/CSS entries from the client reference
manifest, the key will be incorrect and so it won't find the files for
it, resulting in this bug.
### How
This moves the `/` delimiting to be part of the creation of the
`actualSegment` variable so that we only append it when we know we
aren't setting it to be an empty string.
Fixes #52245
Fixes #49871
Closes NEXT-2309
Closes NEXT-2310
