Allow node/entity metatag field empty values to override defaults

Problem/Motivation

Currently, when a content editor editor is editing a node and purposefully removes a metatag field value for that node, the empty value does not get saved and does not override the configured default value.

Steps to reproduce

1. Configure global metatag settings and enter a value for description (or any other field)
2. Edit or create a node
3. Open the Meta tags details and delete the contents in the Basic tags description.
4. Save the node
5. Note that the description metatag in the node page still shows the value from the global configuration

Proposed resolution

1. In the massageFormValues() method of Drupal\metatag\Plugin\Field\FieldWidget\MetatagFireHose,, don't remove empty values for individual fields
2. In the preSave() method of Drupal\metatag\Plugin\Field\FieldTypeMetatagFieldItem, update code so that the entity tag field value is removed from the serialized string when either a default value exists and doesn't match or the default value doesn't exist but the entity value is empty.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet