Fix DataReferenceBase PHPStan L0 issues

Can we just change getString() now, given this is not actually implemented in core?

We could just delete the getString() method from DataReferenceBase, and fall back to the parent implementation?

Works for me, given that the existing method would cause a fatal error.

Something could extend DataReferenceBase in contrib and then implement getType(). This is not that farfetched because DataReferenceBase is abstract.

I think we should do something like:

  /**
   * {@inheritdoc}
   */
  public function getString() {
    if (!method_exists($this, 'getType')) {
      throw new \BadMethodCallException(__CLASS__ . '::getType() not implemented');
    }
    return (string) $this->getType() . ':' . $this->getTargetIdentifier();
  }

We can add a test for this in \Drupal\KernelTests\Core\TypedData\TypedDataDefinitionTest::testDataReferences as we have \Drupal\Core\TypedData\Plugin\DataType\LanguageReference in core that extends but does not implement this method.

@mondrake I am not so sure: https://github.com/phpstan/phpstan/issues/323 implies that method_exists() checks inside the same method will work.

Looks great!

Committed 4695b21 and pushed to 10.1.x. Thanks!
Committed 5615ede and pushed to 10.0.x. Thanks!