fix(connector): add nullable for sentinel_name in redis schema

apps/emqx_connector/src/emqx_connector_redis.erl

@@ -130,6 +130,7 @@ redis_sentinel_fields() ->
     ].
 
 sentinel_name(type) -> binary();
+sentinel_name(nullable) -> true;
 sentinel_name(_) -> undefined.
 
 redis_type(type) -> hoconsc:enum([single, sentinel, cluster]);