uucore: fall back to '~' when the backup suffix is empty

[Adel-Ayoub]

Problem

GNU coreutils treats an empty backup suffix as "unset" and falls back to the
default ~. Our determine_backup_suffix() returned the empty string instead,
so with --backup and an empty --suffix= (or SIMPLE_BACKUP_SUFFIX=) the
backup path became identical to the destination. As a result cp, install
and mv silently created no backup, and ln failed outright.

Reported in #13168.

Before / after (vs GNU coreutils 9.11, LANG=C)

$ echo new > a; echo old > b
$ cp --backup a b --suffix=

command	GNU 9.11	uutils before	uutils after
cp --backup a b --suffix=	b~ (old)	no backup	b~ (old)
install --backup a b --suffix=	b~ (old)	no backup	b~ (old)
mv --backup --suffix= a b	b~ (old)	no backup	b~ (old)
ln --backup --suffix= a b	b~ + link	failed to create hard link 'a' => 'b': Already exists (exit 1)	b~ + link

The precedence --suffix > SIMPLE_BACKUP_SUFFIX > ~ was already correct;
only the empty-suffix fallback was missing.

Fix

determine_backup_suffix() already maps a suffix containing / to the default
~. Extend that check to also cover the empty suffix, at the single shared
location used by cp, install, ln and mv.

Tests

• uucore: test_suffix_empty_defaults_to_tilde
• cp: test_cp_arg_suffix_empty_defaults_to_tilde
• install: test_install_backup_empty_suffix_defaults_to_tilde
• ln: test_ln_backup_empty_suffix_defaults_to_tilde (covers the case that previously failed outright)

Note: the issue also mentions --backup=nil; that part is not a bug (nil is
the alias for existing, and both GNU and uutils already create the backup),
so this PR only addresses the empty-suffix case.

Fixes #13168