sync: fall back to full sync when --file-system has no operands

[can1357]

uutils sync --file-system with no file operands enters the filesystem-sync path with an empty list and returns success without issuing any sync syscall. GNU treats this mode as a global sync() when no operands are given.

Reproduction Steps

Kind of hard to observe.

Impact

Scripts that rely on sync --file-system success as a durability barrier can proceed without flushing data, risking data loss and GNU incompatibility.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!