Allow AIRFLOW_VAR_DEV_LIMIT to be undefined

When I removel `AIRFLOW_VAR_DEV_LIMIT` from my `.env` I got a DAG load
error when Airflow started up because it was trying to turn the empty
string into an int.

rialto_airflow/dags/harvest.py

@@ -18,9 +18,10 @@
 sul_pub_key = Variable.get("sul_pub_key")
 
 # to artificially limit the API activity in development
-dev_limit = Variable.get("dev_limit", default_var=None)
-if dev_limit is not None:
-    dev_limit = int(dev_limit)
+try:
+    dev_limit = int(Variable.get("dev_limit", default_var=None))
+except ValueError:
+    dev_limit = None
 
 
 @dag(