"'if': 'COND'" generates "#if COND". We reject empty COND because it
won't compile. Blank COND won't compile any better, so reject that,
too.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-12-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>