systemctl: we can use nulstr_contains() for this lookup

It's a bit easier to read...
systemctl: make list-dependencies default to default.target
strv: make strv_extend() smarter
systemctl: we can make this faster and shorten it a bit with strv_extend()

Now that strv_extend() is not so slow anymore, we can make use of it, to
shorten our code a bit.
systemctl: no need to check this explicitly, we already checked it a few lines up anyway, so let's just assert
systemctl: it's probably a good idea not to alter return parameters if we fail

We generally follow the rule not to touch return values unless we
succeed, so for the sake of uniformity do the same here.