bug symfony#6270 [Validator] Add warning for null and blank values for minimum string length (theofidry)

This PR was submitted for the 2.3 branch but it was merged into the 2.7 branch instead (closes symfony#6270).

Discussion
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[Validator] Add warning for null and blank values for minimum string length

| Q | A |
| --- | --- |
| Doc fix? | yes |
| New docs? | no |
| Applies to | 2.3+ |
| Fixed tickets | symfony/symfony#16157 |

Follow up of symfony#5789.

Summary of what has been said: even though the [Length#min](http://symfony.com/doc/current/reference/constraints/Length.html#min) option is well detailed and mention the case of `null` values, the introduction is still confusing:

> Validates that a given string length is between some minimum and maximum value.

As a result, a warning note is being added under this introduction.

I've listed all the Symfony constraints. For some this `null` and `blank` values specificity has already been mentioned. I've noted the ones on which this warning does not apply, but for some I am not sure so it needs to be doubled checked.
- Basic Contraints
  - [x] NotBlank: already done
  - [x] Blank: already done
  - [x] NotNull: already done
  - [x] IsNull: already done
  - ~~[ ] IsTrue~~
  - ~~[ ] IsFalse~~
  - ~~[ ] Type~~
- String Constraints¶
  - ~~[ ] Email~~
  - [x] Length
  - ~~[ ] Url~~
  - ~~[ ] Regex~~
  - ~~[ ] Ip~~
  - ~~[ ] Uuid~~
- Number Constraints¶
  - [ ] Range
- Comparison Constraints¶
  - ~~[ ] EqualTo~~
  - ~~[ ] NotEqualTo~~
  - ~~[ ] IdenticalTo~~
  - ~~[ ] NotIdenticalTo~~
  - [ ] LessThan: needed? How `@Assert\LessThan("-18 years")` behaves if the value is null?
  - [ ] LessThanOrEqual: same as `LessThan`
  - [ ] GreaterThan: same as `LessThan`
  - [ ] GreaterThanOrEqual: same as `LessThan`
- Date Constraints¶
  - ~~[ ] Date~~
  - ~~[ ] DateTime~~
  - ~~[ ] Time~~
- Collection Constraints¶
  - ~~[ ] Choice~~
  - ~~[ ] Collection~~
  - ~~[ ] Count~~
  - ~~[ ] UniqueEntity~~
  - ~~[ ] Language~~
  - ~~[ ] Locale~~
  - ~~[ ] Country~~
- File Constraints¶
  - ~~[ ] File~~
  - ~~[ ] Image~~
- Financial and other Number Constraints¶
  - ~~[ ] Bic~~
  - ~~[ ] CardScheme~~
  - ~~[ ] Currency~~
  - ~~[ ] Luhn~~
  - ~~[ ] Iban~~
  - ~~[ ] Isbn~~
  - ~~[ ] Issn~~
- Other Constraints¶
  - ~~[ ] Callback~~
  - ~~[ ] Expression~~
  - ~~[ ] All~~
  - ~~[ ] UserPassword~~
  - ~~[ ] Valid~~

Commits
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38d44c2 [Validator] Add warning for null and blank values for minimum string length

Author: weaverryan

reference/constraints/Length.rst

@@ -1,8 +1,12 @@
 Length
 ======
 
-Validates that a given string length is *between* some minimum and maximum
-value.
+Validates that a given string length is *between* some minimum and maximum value.
+
+**Warning**: *null* and *blank* values are not handled by this constraint. To
+handle them, refers to the :doc:`/reference/constraints/Null`,
+:doc:`/reference/constraints/NotNull`, :doc:`/reference/constraints/Blank` and
+:doc:`/reference/constraints/NotBlank` constraints.
 
 +----------------+----------------------------------------------------------------------+
 | Applies to     | :ref:`property or method <validation-property-target>`               |