replication: use empty password by default

Replication's applier encoded an auth request with exactly the
same parameters as extracted by the URI parser. I.e. when no
password was specified, the parser returned it as NULL, and it was
not encoded. The relay, received such an auth request, complained
that IPROTO_TUPLE field is not specified (this is password).

Such an error confuses - a user didn't do anything illegal, he
just used URI like 'login@host:port', without a password after the
login.

The patch makes the applier use an empty string as a default
password.

An alternative was to force a user always set a password even if
it is an empty string, like that: 'login:@host:port'. And if a
password was not found in an auth request, then reject it with a
password mismatch error. But in that case a URI of kind
'login@host:port' becomes useless - it can never pass. In
addition, netbox already uses an empty string as a default
password. So the only way to make it consistent, and don't break
anything - repeat netbox logic for replication URIs.

Closes #4605

Conflicts:
	test/replication/suite.cfg