[regression] Add a fallback in DraggableListBuilder::getWeight() to support entities with a NULL weight

Problem/Motivation

#2989889: Extract DraggableListBuilderTrait from DraggableListBuilder for use with content entities caused a regression for entities with a NULL weight, as DraggableListBuilder::getWeight() is type-hinted to return an int|float value, but it returns NULL in that case.

Steps to reproduce

1. Create a new taxonomy vocabulary (so that the drag-and-drop interface gets enabled), let's say you called it foo
2. Run drush cset taxonomy.vocabulary.foo weight --input-format=yaml '~'
3. Visit the vocabulary overview

See the following exception:

TypeError: Drupal\Core\Config\Entity\DraggableListBuilder::getWeight(): Return value must be of type int|float, null returned in Drupal\Core\Config\Entity\DraggableListBuilder->getWeight() (line 39 of core/lib/Drupal/Core/Config/Entity/DraggableListBuilder.php).

Proposed resolution

Add a fallback to 0 in DraggableListBuilder::getWeight().

Remaining tasks

Review MR.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

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