Fix DDL update from 0.7 for MS SQL Server

SQL/mssql.upgrade.sql

@@ -246,8 +246,12 @@ GO
 
 -- Updates from version 0.7
 
+ALTER TABLE [dbo].[contacts] DROP CONSTRAINT [DF_contacts_email]
+GO
 ALTER TABLE [dbo].[contacts] ALTER COLUMN [email] [text] COLLATE Latin1_General_CI_AI NOT NULL
 GO
+ALTER TABLE [dbo].[contacts] ADD CONSTRAINT [DF_contacts_email] DEFAULT ('') FOR [email]
+GO
 
 -- Updates from version 0.8-rc