Add fix for versions that contain the NULL string.

sql/patch_39_40_c.sql

@@ -7,7 +7,7 @@
 
 ALTER TABLE xref CHANGE COLUMN version version VARCHAR(10) DEFAULT '0' NOT NULL;
 
-UPDATE xref SET version = '0' WHERE version = NULL OR version = '';
+UPDATE xref SET version = '0' WHERE version = NULL OR version = '' OR version = "NULL";
 
 # patch identifier
 INSERT INTO meta (meta_key, meta_value) VALUES ('patch', 'patch_39_40_c.sql|xref_version');