agent: Allow TERM="".
* agent/call-pinentry.c (start_pinentry): When TERM is none, don't send OPTION ttytype to pinentry.
(cherry picked from commit 0076bef2026a87c4c0e05bad7d322638b1de3f37)
- GnuPG-bug-id: T4137
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>