array_cat null reasoning

Hi,

I'm a bit confused as to the logic of nulls. I understand that null
is to represent an unknown value. So it makes sense that the result
of tacking an unknown value onto a known one is unknown, because you
don't know what exactly you just tacked on. So

select null::text || 'hello';

...returning NULL makes sense. But then why doesn't

select array_cat(null::integer[], '{3}'::integer[]);

...also return null? Somehow it's known what the result is when
combining an unknown array with a known one, but not when combining an
unknown text with a known one? Doesn't this seem inconsistent? If it
does seem inconsistent, should I be careful how to code because this
might change in the future?

Thanks,
Kev