minor #11886 [Validator] Documented the allowEmptyString option in Length (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] Documented the allowEmptyString option in Length

This fixes #11875.

When upmerging to `master`, contents will be replaced by the following:

```rst
allowEmptyString
~~~~~~~~~~~~~~~~

**type**: ``boolean``  **default**: ``false``

If set to ``true``, empty strings are considered valid (which is the same
behavior as previous Symfony versions). The default ``false`` value considers
empty strings not valid.
```

Commits
-------

c510abb [Validator] Documented the allowEmptyString option in Length

Author: javiereguiluz

reference/constraints/Length.rst

@@ -3,15 +3,10 @@ Length
 
 Validates that a given string length is *between* some minimum and maximum value.
 
-.. caution::
-
-    ``null`` and empty strings are not handled by this constraint. You need to
-    also add the :doc:`/reference/constraints/NotBlank` or :doc:`/reference/constraints/NotNull`
-    constraints to validate against these.
-
 ==========  ===================================================================
 Applies to  :ref:`property or method <validation-property-target>`
-Options     - `charset`_
+Options     - `allowEmptyString`_
+            - `charset`_
             - `charsetMessage`_
             - `exactMessage`_
             - `groups`_
@@ -115,6 +110,20 @@ and "50", you might add the following:
 Options
 -------
 
+allowEmptyString
+~~~~~~~~~~~~~~~~
+
+**type**: ``boolean``  **default**: ``true``
+
+.. versionadded:: 4.4
+
+    The ``allowEmptyString`` option was introduced in Symfony 4.4.
+
+When using the ``min`` option, it's mandatory to also define this option. If
+set to ``true``, empty strings are considered valid (which is the same behavior
+as previous Symfony versions). Set it to ``false`` to consider empty strings not
+valid.
+
 charset
 ~~~~~~~