erofs-utils: tar: handle empty filenames correctly

erofs-utils: tar: handle empty filenames correctly

Tar entries with empty filenames are unusual but shouldn't
cause a crash.  Handle this by following `tar` behavior:
substitute `.` for the empty filenames.

Reproducible image (base64-encoded gzipped blob):
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KA9MDAwMDc3VQDShuamBiAaBGA0hGNoaGBgZGJsaAZUaADiGDIomNLBbQylxSWJRUCnlG
Tm4lVXnpGamoNHHtVTClR14ygYBaNgFNAAAAAE6urMAAYAAA==

Fixes: 95d315fd7958 ("erofs-utils: introduce tarerofs")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250228045426.81099-1-hsiangkao@linux.alibaba.com