fix handling of nullable XML attributes

As @Tobion pointed out in #11394, true and 1 are valid values in
boolean XML attributes. The XmlFileLoader didn't handle 1 values
properly.
Merge branch '2.3' into 2.4

* 2.3:
  [DependencyInjection] fix @return anno created by PhpDumper
  Fixed the phpdoc of the VoterInterface
  [DoctrineBridge] Fix empty parameter logging in the dbal logger
  Fixed #11675 ValueToDuplicatesTransformer accept "0" value
  check for the correct field type
  fix handling of nullable XML attributes
  [DomCrawler] fix the axes handling in a bc way

Conflicts:
	src/Symfony/Component/DomCrawler/Tests/CrawlerTest.php
Merge branch '2.4' into 2.5

* 2.4:
  [DependencyInjection] fix @return anno created by PhpDumper
  Fixed the phpdoc of the VoterInterface
  [DoctrineBridge] Fix empty parameter logging in the dbal logger
  Fixed #11675 ValueToDuplicatesTransformer accept "0" value
  check for the correct field type
  fix handling of nullable XML attributes
  [DomCrawler] fix the axes handling in a bc way

Conflicts:
	src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9.php
	src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services9_compiled.php
Merge branch '2.5'

* 2.5:
  [DependencyInjection] fix @return anno created by PhpDumper
  Fixed the phpdoc of the VoterInterface
  [DoctrineBridge] Fix empty parameter logging in the dbal logger
  [Validator] remove duplicate interface implementations
  [Validator] fix return doc of ClassMetadata::getCascadingStrategy
  Fixed #11675 ValueToDuplicatesTransformer accept "0" value
  check for the correct field type
  fix handling of nullable XML attributes
  [DomCrawler] fix the axes handling in a bc way