Map data definition properties are not validated if root value is empty

Problem/Motivation

If a map data definition has required properties but is given an empty array as
its value, the validator skips it entirely and reports no violations. The
required properties are never checked because
RecursiveContextualValidator::validateNode() guards recursion with
!$data->isEmpty(), and an empty map is considered empty regardless
of whether a value was explicitly provided.

The root cause is that Map::$values defaulted to [],
making a map that was never given a value indistinguishable from one
explicitly set to []. Both returned TRUE from
isEmpty(), so both were silently skipped.

Steps to reproduce

use Drupal\Core\TypedData\DataDefinition;
use Drupal\Core\TypedData\MapDataDefinition;

$typed_data_manager = \Drupal::typedDataManager();

$definition = MapDataDefinition::create()
  ->setPropertyDefinition('foo', DataDefinition::create('string')
    ->setRequired(TRUE))
  ->setPropertyDefinition('bar', DataDefinition::create('string')
    ->setRequired(TRUE));

$invalid_but_passes = $typed_data_manager->create($definition, []);
// Should report two violations (foo and bar are missing).
// Actually reports 0.
count($invalid_but_passes->validate()); // 0

Proposed resolution

Two coordinated changes:

1. Change Map::$values to default to NULL.
This lets the validator tell apart a map that was never given a value
(NULL, should be skipped) from one explicitly given an empty array
([], must validate its required properties). FieldItemBase
overrides the default back to [] so the field layer is
completely unaffected.

2. Update RecursiveContextualValidator::isEmptyMapWithValue().
Instead of a separate hasValue flag, the helper now checks

$data->getDataDefinition() instanceof MapDataDefinition &&
   $data->getValue() !== NULL

. Using the definition type rather than
the class avoids importing the field API into the validator: field items
use FieldItemDataDefinition, which does not extend
MapDataDefinition, so they fall out of the check naturally.

EntityUntranslatableFieldsConstraintValidator also receives a
NULL guard (matching the pattern of all other entity constraint
validators), since recursing into a map whose entity property is missing now
instantiates that property with NULL.

Remaining tasks

None.

User interface changes

None.

API changes

Map::getValue() now returns NULL instead of
[] when no value has been set on the map. Code that compared the
return value to [] should be updated to check
!== NULL instead. Field items are not affected. See the change
record for details.

Data model changes

None.

Release notes snippet

Typed data maps now validate their required properties when given an empty
array. Before, an empty map passed validation even when required properties
were missing. Map also defaults to NULL instead of an empty array, so a map
created without a value now returns NULL from getValue(). Field items are not
affected, but maps nested inside field items are, such as the options property
on link fields. See the change record for what to update.