Fix Board settings warning by normalizing size default value

Problem/Motivation

Visiting the Board configuration form triggers the warning:

Warning: foreach() argument must be of type array|object, null given in Drupal\Component\Utility\NestedArray::mergeDeepArray() (line 330 of core/lib/Drupal/Component/Utility/NestedArray.php).

The issue is size can be stored as null or an array, but the select element expects a scalar. This causes a form default merge warning during build.

Proposed resolution

Normalize size to a string (default square) and unwrap arrays before using it as #default_value.

I have attached a patch file for a fix.