minor #11091 Remove allowNull option (julienfalque)

This PR was merged into the 4.2 branch.

Discussion
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Remove `allowNull` option

The option `allowNull` for constraint `NotBlank` is introduced in Symfony 4.3 but appears in the documentation of version 4.2, which can be confusing.

Commits
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c688d26 Remove `allowNull` option

Author: javiereguiluz

reference/constraints/NotBlank.rst

@@ -2,15 +2,13 @@ NotBlank
 ========
 
 Validates that a value is not blank - meaning not equal to a blank string,
-a blank array, ``false`` or ``null`` (null behavior is configurable). To check
-that a value is not equal to ``null``, see the
-:doc:`/reference/constraints/NotNull` constraint.
+a blank array, ``false`` or ``null``. To check that a value is not equal to
+``null``, see the :doc:`/reference/constraints/NotNull` constraint.
 
 +----------------+------------------------------------------------------------------------+
 | Applies to     | :ref:`property or method <validation-property-target>`                 |
 +----------------+------------------------------------------------------------------------+
-| Options        | - `allowNull`_                                                         |
-|                | - `message`_                                                           |
+| Options        | - `message`_                                                           |
 |                | - `payload`_                                                           |
 +----------------+------------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\NotBlank`          |
@@ -83,18 +81,6 @@ class were not blank, you could do the following:
 Options
 -------
 
-allowNull
-~~~~~~~~~
-
-**type**: ``bool`` **default**: ``false``
-
-If set to ``true``, ``null`` values are considered valid and won't trigger a
-constraint violation.
-
-.. versionadded:: 4.3
-
-    The ``allowNull`` option was introduced in Symfony 4.3.
-
 message
 ~~~~~~~