[DomCrawler] fix the axes handling in a bc way

The previous fix in #11548 for handling XPath axes was not backward
compatible. In previous Symfony versions the Crawler handled nodes
by holding a "fake root node". This must be taken into account when
evaluating (relativizing) XPath expressions.
Fixed #11675 ValueToDuplicatesTransformer accept "0" value

Fixed wrong return null syntax

Added transformation to null on empty arrays

Removed useless statement in condition and switched to yoda condition
bug #11676 [Form] Fixed #11675 ValueToDuplicatesTransformer accept "0" value (Nek-)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11676).

Discussion
----------

[Form] Fixed #11675 ValueToDuplicatesTransformer accept "0" value

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #11675
| License       | MIT

Commits
-------

31d48ab Fixed #11675 ValueToDuplicatesTransformer accept "0" value
bug #11624 [DomCrawler] fix the axes handling in a bc way (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[DomCrawler] fix the axes handling in a bc way

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11503
| License       | MIT
| Doc PR        |

The previous fix in #11548 for handling XPath axes was not backward compatible. In previous Symfony versions the Crawler handled nodes by holding a "fake root node". This must be taken into account when evaluating (relativizing) XPath expressions.

Commits
-------

d26040f [DomCrawler] fix the axes handling in a bc way