combine removes filename whitespace when the base is empty #603

fs.path.combine() removes leading whitespace from a valid relative filename when the base is empty:

from fs.path import combine

combine("", " report.txt")  # actual: "report.txt"; expected: " report.txt"
combine("folder", " report.txt")  # "folder/ report.txt"

The same issue affects MemoryFS.walk.files(path=""): a file stored as " report.txt" is returned as "report.txt", so the returned path no longer addresses the file.

The empty-base branch calls str.lstrip() without a character argument. Returning the supplied path unchanged preserves its whitespace and the existing leading-slash behavior. I have a small fix with path and MemoryFS traversal regressions ready. Both regressions fail against upstream; all 2656 project tests pass with the fix on Python 3.10.20 (26 existing skips).

Prepared with AI assistance and independently reviewed before submission.