How to recognize an unmarked checkbox with php and WordPress

I have a problem, when the condition is met, $field_id_value == "1" the checkbox is checked, but when unchecking it, php doesn’t recognise it as unchecked, it is always checked.

I have used if isset() and empty() but it doesn’t detect it

     function wpdocs_unsub_page_callback() {
             ?>
    
                      <form method="post" action="<?php echo esc_html('?page=categoriasrelacionadas'); ?>">
         <div>
                        <input type="button" onclick="checkAll()" value="CHECK ALL">
                        <input type="reset" value="UNCHECK ALL">
                    </div>
                  <?php
        $cat_argtos = array(
          'orderby' => 'name',
          'order'   => 'ASC'
        );
        $categorias = get_categories($cat_argtos);
        foreach($categorias as $categoria) {
          $args = array(
            'showposts' => -1,
            'category__in' => array($categoria->term_id),
            'caller_get_posts'=>1
          );
          $entradas = get_posts($args);
            
          $field_id_value = get_option($categoria->term_id);
          if($field_id_value == "1"){
$field_id_checked = 'checked="checked"';
} else {
$field_id_checked=" ";
}
          echo '<p>' .$categoria->name.'<input id="myCheck'.$categoria->term_id.'" type="checkbox" '.$field_id_checked.' class="check3" name="categorias[]"  value="'.$categoria->term_id.'" />  </p> ';
        }

             ?>
            
                        </p>
                <!-- #universal-message-container -->
         
                <?php
                    wp_nonce_field( 'acme-settings-save', 'acme-custom-message' );
                    submit_button();
        
                ?>
        
            </form>
        
        
        
             <?php 
            if(isset($_POST['submit'])){//Validacion de envio de formulario

             if (!isset($_POST['categorias'])) {
            
                 $checked = 0;
            }
             else {  $checked = 1;} 
        foreach($_POST['categorias'] as $selected){
            
            update_option($selected, $checked);
        echo $selected."</br>";// Imprime resultados
        }
        
        }
             
         }