How to write an if condition for a radio button?

Question

PHP

Greetings!
The field in the form is:

<input type="text" id="color_dunkle" name="color_dunkle" class="text" value="{$editable_page.editable_page_theme_config.color_dunkle|default=''}">

When creating new content and when editing already created content, input with

{$editable_page.editable_page_theme_config.color_dunkle|default=''}

works without errors.
If new content is being created, then the value of the color_dunkle parameter is omitted and |default='' is then fired.

I want to convert c input into radio buttons.
To do this, you need to write conditions for the radio buttons.
I wrote such a condition, using the example of one button I will show:

<label for="article_show1" class='{if condition="$editable_page.editable_page_theme_config.color_dunkle eq 1"}selected{/if}'>Да</label>

When editing previously created content - this button works well because the value of the variable has already been created.
But if I want to create new content, I get a 500 error because the value of the variable doesn't exist and I can't attach |default='' to the end of the variable.
How do I attach |default='' to the end of a variable?
It should be like this:

{if condition="$editable_page.editable_page_theme_config.color_dunkle|default='' eq 1"}selected{/if}

Of course, this is not correct and does not work that way, but I do not know how to solve this error correctly.
Thanks in advance to those who respond.