minor #13668 [Validator] clarify empty string behavior without lower bound given (xabbuh)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] clarify empty string behavior without lower bound given

see symfony/symfony#36812

Commits
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6d951ec clarify empty string behavior without lower bound given

Author: javiereguiluz

reference/constraints/Length.rst

@@ -128,6 +128,10 @@ 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.
 
+.. caution::
+
+    This option does not have any effect when no minimum length is given.
+
 charset
 ~~~~~~~