minor #7775 #7311 choice_value callback argument can be null (Michael Lenahan, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7775).

Discussion
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#7311 choice_value callback argument can be null

This is my first contribution to Symfony. Thank you @Deltachaos for your help today at the Hackday in Cologne.

Commits
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bf604e5 Fixed a syntax issue
da1bbb2 Use backticks
c274800 #7311 choice_value callback argument can be null

Author: xabbuh

reference/forms/types/options/choice_value.rst.inc

@@ -14,6 +14,9 @@ configure the value that will be sent in the API request).
 This can be a callable or a property path. See `choice_label`_ for similar usage.
 If ``null`` is used, an incrementing integer is used as the name.
 
+If you are using a callable to populate ``choice_value``, you need to check
+for the case that the value of the field may be ``null``.
+
 .. caution::
 
     In Symfony 2.7, there was a small backwards-compatibility break with how the