how function-based index buttress contains nulls seer 10+?

lets only contend have list oracle:

create list chairman (
  id array primary key,
  given_names varchar2(50),
  surname varchar2(50)
);

with function-based indices:

create index idx_person_upper_given_names chairman (upper(given_names));
create index idx_person_upper_last_name chairman (upper(last_name));

now, given_names zero values nonetheless argument's effect last_name does. i this:

select * chairman where upper(given_names) 'p%'

the explain digest tells me the controlling index nonetheless change to:

select * chairman where upper(last_name) 'p%'

it doesn't. seer docs contend function-based index wholly used several conditions met, ensuring there zero values given aren't indexed.

i've attempted queries:

select * chairman where upper(last_name) 'p%' upper(last_name) null

and

select * chairman where upper(last_name) 'p%' last_name null

in latter box i even total an index last_name nonetheless matter i try uses full list scan. presumption i can't absolved zero values, i query index upper(last_name)?