`1x` should output empty line if input doesn't contain newline

GNU `sed` outputs an empty line with `1x` if the input doesn't contain a newline. uutils `sed`, on the other hand, doesn't:
```bash
$ printf "abc" | sed 1x

$ 
$ printf "abc" | cargo run -q sed 1x
$
```
The exception is an empty string as input: in such a case there is no output.
```bash
$ printf "" | sed 1x
$ 
$ printf "" | cargo run -q sed 1x
$
```