fix db migration add filters

src/main/resources/db/migration/V3_0__AddFilter.sql

@@ -3,6 +3,6 @@ ALTER TABLE query ADD COLUMN sql_filter text;
 ALTER TABLE query_filter DROP COLUMN filters_id;
 DROP TABLE filter_values, filter;
 
-ALTER TABLE query_filter ADD COLUMN code character varying(255) NOT NULL;
+ALTER TABLE query_filter ADD COLUMN code character varying(255) NOT NULL DEFAULT '';
 ALTER TABLE ONLY query_filter
     ADD CONSTRAINT fk_1 FOREIGN KEY (query_id) REFERENCES query(id);
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