The old regex requires something after the 'host' part. Fix this.

Old regex is : `/^GRANT\s(.+)\sON\s(.+)\sTO\s(.*)@(.*?)(\s.*)$/` . The
last part `(\s.*)$` means "a space followed by anything". The issue is
that when user has no `GRANT` privileges, the `"SHOW GRANTS FOR #{user_string}"` returns
```sql
GRANT SELECT ON `database`.* TO 'user'@'%'
```
which does not match `(\s.*)$` .
This small patch fixes this making last block optional (thanks to `?`).