python: Fix configure generating setup.py.
* lang/python/setup.py.in: Handle the case, when substitutions may be empty.
- Reported-by: Andreas Metzler
- Fixes-commit: 718981a53972b35bb068b55fd139b751fadc0157
- GnuPG-bug-id: T6204
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>