minor #6922 [Config] Add ExprBuilder::ifEmpty() (ogizanagi)

This PR was merged into the master branch.

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[Config] Add ExprBuilder::ifEmpty()

After symfony/symfony#19764

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f0fe739 [WCM][Config] Add ExprBuilder::ifEmpty()

Author: weaverryan

components/config/definition.rst

@@ -736,6 +736,7 @@ the following ways:
 - ``ifTrue()``
 - ``ifString()``
 - ``ifNull()``
+- ``ifEmpty()`` (since Symfony 3.2)
 - ``ifArray()``
 - ``ifInArray()``
 - ``ifNotInArray()``