do not write user/password info to y2log (bnc #893362)

> So far I have tested it's checked whether both values (user and password) are set.
But nevertheless isn't it more secure to have a default?

We may be getting out of scope of this PR, but anyway:

On the *immediately preceding line* we check that `incoming` is not empty. Using a default in that case is pointless. And I have checked, the only caller of this function either passes *both* elements of the pair or *none* so the default of `incoming/outgoing[1]` can be dropped too.
Just add a guard at the method start:
```ruby
raise ArgumentError, "incoming must be a pair, optionally empty" unless incoming.empty? || incoming.size == 2
raise ArgumentError, "outgoing must be a pair, optionally empty" unless outgoing.empty? || outgoing.size == 2
```

(Regarding "security" of this kind, Avdi Grimm's Confident Ruby has helped me clarify things. This is 3.12 Reject unworkable values with preconditions)


Also note that with string interpolation (`#{foo}`) an implicit `to_s` is called, and `nil.to_s` is the empty string anyway.