[PHP 8.5] OptionsWidgetBase::getSelectedOptions tries to access null array key in some scenarios

Problem/Motivation

Originates from #3576485-7: PHP8.5 deprecation when using empty list items

All list_* widgets in core (OptionsSelectWidget, OptionsButtonsWidget) declare multiple_values: TRUE.
But if we try to define a widget without it, as done in the scheduler_content_moderation_integration contrib module, we get the following deprecation:

Using null as an array offset is deprecated, use an empty string instead in /var/www/html/web/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php on line 190

In such case, WidgetBase::form() routes through formMultipleElements() instead of formSingleElement(). For a field with no stored value, formMultipleElements() calls $items->appendItem() to fill the first delta slot, producing an item with value = null. OptionsWidgetBase::getSelectedOptions() then iterates all items and evaluates isset($flat_options[null]);

Steps to reproduce

- Run Drupal (main branch) with PHP 8.5
- Define a FieldWidget plugin extending OptionsWidgetBase without declaring multiple_values: TRUE in the plugin attribute (see test in MR for example)
- Attach that widget to a list_* field on a content type
- Go to the node create form - /node/add/[content_type] > We see the PHP 8.5 deprecation warning

Proposed resolution

Guard OptionsWidgetBase::getSelectedOptions with a null check:

// line 190
- if (isset($flat_options[$value])) {
+ if ($value !== NULL && isset($flat_options[$value])) {

Remaining tasks

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User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

None