qapi: Reject blank 'if' conditions in addition to empty ones

"'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>
3 files changed