Improper sed -i Behavior

When -i is provided without an attached suffix, uutils/sed incorrectly consumes the subsequent positional argument (the sed expression) as the backup suffix. This causes the target filename to be evaluated as a sed script, resulting in parsing errors.
In GNU sed, the suffix for -i is optional and must be concatenated directly without whitespace

Reproduction Steps:

echo "foo" > test.txt
sed -i 's/foo/bar/' test.txt

Observed Output:

sed: <script argument 1>:1:1: error: undefined label `est.txt'

Expected Output:

Exit code 0, test.txt modified in-place with contents "bar", no backup file created.

Start by tracing uutils/sed's command-line parsing for -i when no suffix is attached, using the reported shell reproduction as the first check. Done means the following sed expression is no longer consumed as a backup suffix, the command exits successfully, test.txt contains "bar", and no backup file is created.