Entity label EntityBrowserFieldWidgetDisplay plugin should return a render array

Problem/Motivation

When using the entity label option for displaying the selected entities in the field widget, the following error is displayed:

User error: "display" is an invalid render array key in Drupal\Core\Render\Element::children() (line 98 of core/lib/Drupal/Core/Render/Element.php)

The \Drupal\entity_browser\Plugin\EntityBrowser\FieldWidgetDisplayEntityLabel::view method expects to return a render array. Instead, if the entity has a label, it returns the EntityInterface::label method value, which has one of the string|\Drupal\Core\StringTranslation\TranslatableMarkup|null types.

Steps to reproduce

• Select entity browser as field widget when configuring the form display
• When configuring the field widget select "Entity label" as Entity display plugin
• Edit an entity that uses the widget

Proposed resolution

This can be fixed by wrapping the returned string into a render array markup element.