bug #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 #6270).

Discussion
----------

[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 #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>`               |