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