Ignore null value in comparison validators
merged branch EmmanuelVella/validators (PR #8577)

This PR was merged into the 2.3 branch.

Discussion
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Validators

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This PR avoid comparison validators to be executed if the compared value is null.

Commits
-------

48338fc Ignore null value in comparison validators
[Validator] fixed the wrong isAbstract() check against the class (fixed #8589)
corrected English grammar (s/does not exists/does not exist)
Merge branch '2.2' into 2.3

* 2.2:
  corrected English grammar (s/does not exists/does not exist)
  [Process] Add more precision to Process::stop timeout
  [Process] Avoid zombie process in case of unit tests failure
  [Process] Fix #8739
  [Process] Add failing test for #8739
  [Process] Fix CS
  Fixed documentation grammar for AuthenticationManagerInterface::authenticate()
  [Validator] fixed the wrong isAbstract() check against the class (fixed #8589)
  [TwigBridge] Prevent code extension to display warning
  Use strstr instead of strpos

Conflicts:
	src/Symfony/Component/Finder/Shell/Command.php
	src/Symfony/Component/Process/Process.php
Merge branch '2.3'

* 2.3:
  [Process] Revert change
  [Process] Fix #8746 : slowness added in unit tests since #8741
  [Process] Fix #8742 : Signal-terminated processes are not successful
  corrected English grammar (s/does not exists/does not exist)
  [Process] Add more precision to Process::stop timeout
  [Process] Avoid zombie process in case of unit tests failure
  [Process] Fix #8739
  [Process] Add failing test for #8739
  [Process] Fix CS
  [TwigBridge] removed superflous ; when rendering form_enctype() (closes #8660)
  Fixed documentation grammar for AuthenticationManagerInterface::authenticate()
  [Validator] fixed the wrong isAbstract() check against the class (fixed #8589)
  [TwigBridge] Prevent code extension to display warning
  Fix internal sub-request creation
  [FrameworkBundle] made code more generic
  [Form] Moved auto_initialize option to the BaseType
  Use strstr instead of strpos
  Make sure ContextErrorException is loaded during compile time errors
  Fix empty process argument escaping on Windows
  Ignore null value in comparison validators

Conflicts:
	src/Symfony/Component/Debug/Tests/ErrorHandlerTest.php
	src/Symfony/Component/HttpKernel/Tests/Fragment/InlineFragmentRendererTest.php
	src/Symfony/Component/Process/Process.php