Add some smarts when making batch execution form schema

in some cases `has_execution_options()` may return True but the base
view class may not need to provide any options itself (i.e. subclass
is responsible for declaring the view has options).

tailbone/views/batch/core.py

@@ -786,10 +786,12 @@ class BatchMasterView(MasterView):
         route_prefix = self.get_route_prefix()
         use_buefy = self.get_use_buefy()
 
+        schema = None
         if self.has_execution_options(batch):
             if batch is None:
                 batch = self.model_class
             schema = self.make_execute_schema(batch)
+            if schema:
                 for field in schema:
 
                     # if field does not yet have a default, maybe provide one from session storage
@@ -807,7 +809,7 @@ class BatchMasterView(MasterView):
                     if use_buefy and isinstance(field.widget, forms.widgets.PlainSelectWidget):
                         field.widget = dfwidget.SelectWidget(values=field.widget.values)
 
-        else:
+        if not schema:
             schema = colander.Schema()
 
         kwargs['use_buefy'] = use_buefy