Why does this namespace reject need an explicit [' ']or [none] in the setter?

Configuration
1

Continuing the discussion from Allow for multiple labels/areas in label_entities/area_entities:

using this:

{% set areas = {'person.1':'logeerkamer', "person.2":'library',
                'person.3':'voorkamer', "person.4":'dorm' } %}
{% set persons = ["person.1", "person.2","person.3", "person.4"] %}
{% set ns = namespace(occupied = [' ']) %}
{% for present in persons|select('is_state', 'home')  %}
  {% set ns.occupied = ns.occupied + [areas.get(present)] %}
{% endfor %}
{{['dorm','aanbouw','woongedeelte']|map('floor_areas')|sum(start=[])
  |reject('match',ns.occupied|join('|'))
  |map('area_entities')|sum(start=[])
  |select('in',label_entities('binnenlamp'))
  |select('is_state','on')|list|sort}}

makes it possible to map those persons to a room (thank you @hilburn in Discord)

however, there is 1 thing I am not sure to be doing right:
if the reject is empty (so all areas should be mapped) the output of the template is [] if I do Not enter

{% set ns = namespace(occupied = [' '])%}

or set it to [none]

but do as I normally do:

{% set ns = namespace(occupied = [])%}

is this a peculiarity of the reject filter, or why else would this be the case.

2

update

{% set ns = namespace(occupied = []) %}
{% for present in persons|select('is_state', 'home')  %}
{% set ns.occupied = ns.occupied + [areas.get(present)] %}
{% endfor %}
{{['dorm','aanbouw','woongedeelte']|map('floor_areas')|sum(start=[])
|reject('in',ns.occupied)
|map('area_entities')|sum(start=[])
|select('in',label_entities('binnenlamp'))
|select('is_state','on')|list|sort}}
{{ns.occupied}}

changing the reject to

|reject('in',ns.occupied)

makes it happen, and can let the set be {% set ns = namespace(occupied = []) %}

thank once more to @hilburn stating

You can use reject(in, ns.occupied) instead of the regex reject

the check mark for the solution goes to Hilburn