Use 'default' instead of '' as default user name and password

For D7 we are now using '' (empty string) as default user name and password. This issue hope to redefine these empty string as 'default'.

As mentioned in #334687: [DBTNG + XDB] NOT NULL fields using a DEFAULT '' clause , '' (empty string) is not always functioning for all database engine, e.g. Oracle will translate empty string as NULL automatically, which is not ANSI standard. It is also not too meaningful if using empty string as undefined default value. Use 'default' seems to be a possible replacement.