Permit empty passwords in net.box

This is required to do the following:

tarantoolctl user@host
tarantoolctl guest@host
tarantoolctl guest:@host

Currently all of those will lead to error from net.box. Either because
password is not specified, or because guest user can't login with empty
password.

This change effectively sets guest password to an empty string, to make
it less of a special case to authentication.

And since now guest can be authenticated with an empty password, net.box
will replace nil passwords with empty strings, when user name is
provided.

We still support the case when username=nil and password=nil. In this
case net.box doesn't send authentication packet at all. Guest login is
just assumed.

The bootstrap snapshot will now have guest user with an empty password,
instead of no password.

Fixes #1545