Never allow empty passwords (#2116)

Never allow empty passwords (#2116)

Passwords for encryption must not be empty. Neither through command line
option nor through interactive input.

With this PR applied:
```
$ bsdtar --format zip --options zip:encryption -cf archive.zip input.txt
Enter passphrase: <press enter>
bsdtar: Encryption needs passphrase
```

Output with command line argument (unaffected by this PR):
```
$ bsdtar --format zip --options zip:encryption --passphrase '' -cf archive.zip input.txt
bsdtar: Empty passphrase is unacceptable
```

The outputs differ due to internal difference in handling the results.

It is still possible to supply a passphrase through command line
argument which cannot be entered interactively, i.e. $'\r\n'. See
https://github.com/libarchive/libarchive/pull/2115 for more details.