Preserve filename whitespace when combining an empty base

[xiehuanyi]

Fixes #603.

combine("", " report.txt") currently returns "report.txt", removing a character from a valid filename. The same path change occurs during MemoryFS.walk.files(path=""), so its returned path cannot be used to read the original file.

Return the supplied path unchanged when the base is empty. This preserves filename spaces/tabs and the existing behavior for leading slashes. Other path combination behavior is unchanged.

Validation: both new path and MemoryFS traversal regressions fail against upstream. All 2656 project tests pass on supported Python 3.10.20 (26 existing skips); Black 22.3 and flake8 checks pass. The local environment uses setuptools<81 for the existing pkg_resources import and pyftpdlib 1.5.9 within the project's declared range; pyftpdlib 1.5.10's test teardown expects a newer psutil API than the declared psutil 5.x dependency. No dependency files were changed.

Prepared with AI assistance and independently reviewed before submission.

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