[lldb] Fix an asan error from 27df2d9f

This error is caused by a combination of a couple of factors:
- the test accidentally creating a list with a single (empty) FileSpec
  instead of an empty list
- lldb overzeleously converting empty strings into nullptrs
- asan overzeleously validating symlink(2) arguments (the real symlink
  call would just fail with EFAULT)

I fix this by using FileSpec::GetPath instead of GetCString. This avoids
the nullptr and also avoids inserting the path into the global string
pool.

I also enhance the test case to test both empty paths and empty lists.