grant '' to foo@localhost;
ERROR OP000: Invalid role specification ``
create user ''@localhost;
create role r1;
grant r1 to ''@localhost;
connect  con1,localhost,nonexisting_user,,;
select current_user;
current_user
@localhost
show grants;
Grants for @localhost
GRANT `r1` TO ``@`localhost`
GRANT USAGE ON *.* TO ``@`localhost`
connection default;
drop role r1;
drop user ''@localhost;