Fix #77322: PharData::addEmptyDir('/') Possible integer overflow

`phar_path_check()` already strips a leading slash, so we must not
attempt to strip the trailing slash from an now empty

Fix #77322: PharData::addEmptyDir('/') Possible integer overflow

`phar_path_check()` already strips a leading slash, so we must not
attempt to strip the trailing slash from an now empty directory name.

Closes GH-6508.

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