Avoid meaningless flushes within importer main loop

previously, if there were 0 created/updated, it would flush.  now it will only
flush if created or updated are nonempty

rattail/importing/importers.py

@@ -300,7 +300,14 @@ class Importer(object):
                     log.debug("did NOT create new {} for key: {}".format(self.model_name, key))
 
             # flush changes every so often
-            if not self.batch_size or (len(created) + len(updated)) % self.batch_size == 0:
+            flush = False
+            if not self.batch_size or self.batch_size == 1:
+                # flush every time if no meaningful batch size
+                flush = True
+            elif created or updated:
+                if (len(created) + len(updated)) % self.batch_size == 0:
+                    flush = True
+            if flush:
                 self.flush_create_update()
 
         try: