python: fix return value of emptiness_check_instantiator

* wrap/python/spot.i: Fix typemap for
emptiness_check_instantiator::construct.  The previous code used
to turn (None, "error") into simply ("error").
* wrap/python/ajax/spot.in: Fix handling or errors when
instantiating emptiness checks.

wrap/python/ajax/spot.in

@@ -489,6 +489,7 @@ if output_type == 'r':
     if not eci:
         unbufprint('<div class="parse-error">Cannot parse "' + opt
                          + '" near "' + err + '".</div>')
+        ec = 0
     else:
         ec_a = 0
         n_acc = degen.number_of_acceptance_conditions()