gpg: The option --passphrase= can be empty.
* g10/gpg.c (opts): Use ARGPARSE_o_s for oPassphrase to allow empty string.
Cherri-picked from master commit of:
fcd766719a6e8f18f4be4c0f91e12aa157ca5506
- GnuPG-bug-id: T4633
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>