在前端检查未定义的树枝

I'm trying to create an opened/closed shop widget in Twig. This have to be done in the frontend since I don't have access to create backend functions (SaaS platform)

I got almost everything working except for one thing. When you fill in a value that's not equal to one of the statements then the server throws a 500 internal error. Whatever I try I just can't see how to check if that particulary var is set.

To clarify:

I can set these options:

theme.monday
theme.tuesday
theme.wednesday
etc...

Let's say I fill theme.monday with value 09.00-16.00. Which means shop is open from 9am till 4pm. This works with below code. When I fill theme.monday with value CLosED then the code below works as well.

However when I fill theme.monday with value Hello! blabla then the server throws an internal error.

With my limited Twig knowledge I understand that this error comes from the var combine. If something else is set as value then that split function won't work offcourse.

My question is how to handle this? So how can I make some sort of check that will work when theme.monday is filled with something different then closed or 09.00-16.00?

{% set open = false %}
{% set now = 'now' | date('U') %} 
{% set today = 'now' | date('l') | lower %}
{% set closedval = attribute(theme, today) | lower | t  %} 

{% if 'closed' in closedval %}
  {% set open = false %}
{% else %}
  {% set combine = attribute(theme, today) | split('-') %}
  {% set opening_time = combine[0] | date('U') %}
  {% set closing_time = combine[1] | date('U')  %}
  {% if (now < closing_time) and (now > opening_time) %}
    {% set open = true %}
  {% endif %}
{% endif %}

Things I tried are:

{% set open = false %}
{% set now = 'now' | date('U') %} 
{% set today = 'now' | date('l') | lower %}
{% set closedval = attribute(theme, today) | lower | t  %} 

{% set combine = attribute(theme, today) | split('-') %}

{% if 'closed' in closedval %}
  {% set open = false %}
{% elseif combine is not null %} 
{# or 
  {% elseif combine is defined %}
  {% elseif combine is not sameas(false) %}
 #}

  {% set opening_time = combine[0] | date('U') %}
  {% set closing_time = combine[1] | date('U')  %}
  {% if (now < closing_time) and (now > opening_time) %}
    {% set open = true %}
  {% endif %}

{% else %}
  {% set open = false %}
{% endif %}

However above won't work. I can't see what I'm doing wrong. Any help greatly appreciated.