qapi: Generate simpler marshalling code when no arguments

qapi: Generate simpler marshalling code when no arguments

When command FOO has no arguments, its generated qmp_marshal_FOO() is
a bit confusing.  Make it simpler:

     visit_start_struct(v, NULL, NULL, 0, &err);
     if (err) {
         goto out;
     }
-
-    if (!err) {
-        visit_check_struct(v, &err);
-    }
+    visit_check_struct(v, &err);
     visit_end_struct(v, NULL);
     if (err) {
         goto out;
     }

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200424084338.26803-16-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>