Allow empty subject fields in keytool

`keytool -genkeypair` will prompt for the subject name if `-dname` option is not specified. There are several fields (CN, OU, etc) and each has a default value. User either enters a new value or types enter to accept the default value. This means there is no way to bypass a field.

In openssl, "a field can still be omitted if a default value is present if the user just enters the '.' character." We can do the same.

Note that you cannot bypass all fields because the name is also used as the issuer name and it cannot be empty.