edit/add.php how to create "radio" button without any value = none

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CUSTOM BLOCK: I want to use some radio button inside form.php to customize the view.php.

"THE PROBLEM" - i want in the select also the "none" option (var = null).

Inside edit/add.php (the form) i create array match the DB decleraion with option for "space around image" (works great)
<?php
$filterSpaceAroundImageOptions = array(
   'no-space' => t('Image without any spacing'),
   'small-space'  => t('Image with small space around'),
   'big-space'  => t('Image with large space around'),
...rest of the code

Inside db.xml
<field name="filterSpaceAroundImageOption" type="string" size="50" 
      comment="('no-space','small-space', 'big-space')">
         <default value="no-space"/>
      </field>

Inside view.php i have something like this:
<img class="some-class <?php echo filterSpaceAroundImageOption ?>"....rest of the code

(medium / large space) are great, but If the user chose "no-space" the class need to be empty "" (No extra style)

Of course i can add "IF" for "'no-space' " option (or wrote some "un-declare" css class), "but this is not modular or clean, so mabye her i will find good solution for this

siton