header array empty in theme.inc TypeError: strlen(): Argument must be of type string, array given in Unicode.php

Problem/Motivation

TypeError: strlen(): Argument #1 ($string) must be of type string, array given in strlen() (line 395 of core/lib/Drupal/Component/Utility/Unicode.php)

The problem is in template_preprocess_field_multiple_value_form() where there is an empty array in the header.

$header = [
      [
        'data' => [
          '#type' => 'html_tag',
          '#tag' => 'h4',
          '#value' => $element['#title'],
          '#attributes' => $header_attributes,
        ],
        'colspan' => 2,
        'class' => ['field-label'],
      ],
      [], // Problem is here.
      t('Order', [], ['context' => 'Sort order']),
    ];

Steps to reproduce

To be added

Proposed resolution

Remove the cell and increase the colspan of the previous cell.

Merge request link

https://git.drupalcode.org/project/drupal/-/merge_requests/6323

Remaining tasks

Add test coverage

User interface changes

Removal of empty header cell.

API changes

None.

Data model changes

None.

Release notes snippet

TBD