Remove superfluous target_bundles form validation for entity types with bundles

Problem/Motivation

DefaultSelection::validateTargetBundles() converts an empty target bundle selection from [] to NULL, meaning that all bundles are referenceable.

For entity types with bundles, the configuration form however requires at least one target bundle to be selected. Neither [] nor NULL is accepted. As noted in #20, the conversion therefore cannot enable a successful form submission without a selected bundle.

As for the background requested in #9, the #required flag was introduced per #1953444: Make 'target bundles' required on the Entity reference instance settings form. Its purpose was to prevent site builders from overlooking the bundle selection, thereby selecting all bundles by default, even future ones.

Even worse, a semantic that turns [] (no bundles allowed) into NULL (all bundles allowed) is inherently problematic, whenever the last previously selected bundle happens to be deleted from the entity type, as described in #3031637-17: Entity reference: allow specifiying that every bundle is allowed.

For entity types with bundles, the form validator validateTargetBundles() should therefore be deprecated and subsequently removed. For bundleless entity types, the form should supply NULL directly.

This issue does aim to change the handler's default configuration, the bundle selection's #required flag, or the supported NULL/[] semantics.

Steps to reproduce

1. Inspect DefaultSelection::buildConfigurationForm().
2. For entity types with bundles, observe that target_bundles is #required.
3. For bundleless entity types, observe that the form supplies [] through a value element.
4. Observe that both cases register validateTargetBundles(), which converts [] to NULL.

Proposed resolution

• Set the form element's value directly to NULL for bundleless entity types.
• Remove the registration of validateTargetBundles().
• Deprecate the public static method for backward compatibility.

Remaining tasks

Review.

User interface changes

None. Target bundle selection remains #required for entity types with bundles.

API changes

None. The public callback remains available, and programmatic configuration continues to support both NULL and [] for target_bundles.

Data model changes

None. The resulting saved configuration remains unchanged.

Original Report

For security reasons, we took the explicit design decision that if no target bundle is configured, we shouldn't fall back to allowing all bundles.

However, DefaultSelection::validateConfigurationForm() still contains:

    // If no checkboxes were checked for 'target_bundles', store NULL ("all
    // bundles are referenceable") rather than empty array ("no bundle is
    // referenceable" - typically happens when all referenceable bundles have
    // been deleted).
    if ($form_state->getValue(['settings', 'handler_settings', 'target_bundles']) === []) {
      $form_state->setValue(['settings', 'handler_settings', 'target_bundles'], NULL);
    }

This is not a big problem, as the 'target_bundles' form element is #required anyway, so the case that an empty array is in the values to be validated may never occur. Even more, this means these lines should not be there.

This issue was split off #2429699: Add Views EntityReference filter to be available for all entity reference fields: Add Views EntityReference filter to be available for all entity reference fields.