than an empty host '' is the same as any-host wildcard '%'.
Replace '' with '%' in the parser (for GRANT ... foo@'') and when loading grant tables.
Side effect: one cannot have foo@'' and foo@'%' both at the same time
(but one can have foo@'%' and foo@'%%')
configuration wizard to fail
Made the fields mysql.user.plugin and mysql.user.authentication_string
nullable to conform with some older clients doing inserts instead of
using the commands.
