Fix Herald repetition policy migration for NULL

When we change a nullable column to a non-nullable column, we can get a
data truncation error if any value was "NULL".

This is exceptionally unusual, but our two very oldest Herald rules have
a "NULL" policy on `secure`.

resources/sql/autopatches/20180124.herald.01.repetition.sql

@@ -5,6 +5,10 @@
 
 */
 
+UPDATE {$NAMESPACE}_herald.herald_rule
+  SET repetitionPolicy = '1'
+  WHERE repetitionPolicy IS NULL;
+
 ALTER TABLE {$NAMESPACE}_herald.herald_rule
   CHANGE repetitionPolicy
     repetitionPolicy VARCHAR(32) NOT NULL COLLATE {$COLLATE_TEXT};