[#3428049] Fixed D10.2 error

Closes #3428049

On Drupal 10.3.1, PHP 8.3 I'm getting:

TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given in computed_field_plugin_form_alter() (line 80 of modules/contrib/computed_field_plugin/computed_field_plugin.module). 

I would add here default $form['add']['new_storage_type'] ?? [].

Debug shown that this form alter callback called twice:

• first time $form contains form for field type selection
• second time - form for a new field label

Looks like this is a multistep form shares same form id.

How about reducing nesting level by using here early return: form_id !== 'id' { return; }?

I would recommend to use if instead of elseif here and in the about if statement add return statement. This gives easier code refactor when Drupal 10.2+ will be requirement for this module and we want to drop version check statement as old code:

// The "if" statement for the old version completely ready
// for standalone drop in the future code cleanup.
if (version_check($current, $expected, '<')) {
  // Something very useful for old version.
  return;
}

if ($something_useful_for_new_version) {
  // ...
}

Thank you @HitchShock for your work! I discovered only one major issue with php type error on php8.3. Also added some minor suggestions. Otherwise the approach for the issue resolving looks similar to what I saw in other contrib modules.