Media::getName() can return NULL for oEmbed media items

Problem/Motivation

\Drupal\media\Entity\Media::getName() can return NULL if $media_source->getMetadata($this, $media_source->getPluginDefinition()['default_name_metadata_attribute']) returns NULL.

Steps to reproduce

Use the https://www.drupal.org/project/media_entity_instagram module. Link to an instragram with the media system, then delete the instragram. BOOM you'll get an error when trying to edit the media item because Html::escape() does not accept a NULL

Proposed resolution

Fallback to a label based on the media ID and a message about it being broken.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet