Entity reference field missing in Diff output when only target_id is available

Problem/Motivation

When comparing revisions of an entity that contains an Entity Reference field, the Diff module may fail to display reference updates.
This occurs when the field value provides only the target_id (e.g., after an entity reference update) but does not include the loaded entity object.

As a result, the updated reference change is not visible in the Diff output, which can cause confusion for users reviewing content revisions.

Steps to reproduce

Create a content type with an Entity Reference field referencing another entity (e.g., Node or Taxonomy Term).

Create a node and set a reference value.

Edit the node and change the reference to a different entity.

Go to Revisions → Compare revisions (Diff view).

Observe that the entity reference field does not display any change, even though the reference has been updated.

Proposed resolution

Add a fallback condition in EntityReferenceFieldBuilder to handle cases where 'target_id' exists but 'entity' is not available.

This ensures the updated entity reference ID is displayed properly in the Diff output.

Remaining tasks

Add fallback for target_id when entity is missing

Verify that updated entity references appear in revision comparison

Confirm backward compatibility with existing Diff displays

User interface changes

In Diff view, when only the target_id is available, the output will now display a message such as:
Entity ID: 25

API changes

None

Data model changes

None