minor #16743 [ExpressionLanguage] Feature Null-coalescing operator (mytuny)

This PR was merged into the 6.1 branch.

Discussion
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[ExpressionLanguage] Feature Null-coalescing operator

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[ExpressionLanguage] [NEW FEATURE]

This PR introduces the support for null-coalescing operator ``??``.

The proposed change, includes a paragraph under the sub-page /expression_language/syntax to describe the new feature usage. The sub-paragraph has a title of **Null-coalescing operator** under the paragraph **Supported Operators**.

The actual work related to this Doc PR available as Symfony PR <!--symfony/symfony#45795>.

Commits
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81204c3 [ExpressionLanguage] Feature Null-coalescing operator

Author: javiereguiluz

components/expression_language/syntax.rst

@@ -343,6 +343,18 @@ Ternary Operators
 * ``foo ?: 'no'`` (equal to ``foo ? foo : 'no'``)
 * ``foo ? 'yes'`` (equal to ``foo ? 'yes' : ''``)
 
+Null-coalescing Operator
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+This is the same as the PHP's null-coalescing operator ``??`` which is a syntactic sugar for the use of a ternary 
+in conjunction with isset(). It returns the left hand-side if it exist and not ``null`` otherwise it returns the right hand-side.
+Note that coalescing can be chained.
+
+* ``foo ?? 'no'``
+* ``foo.baz ?? 'no'``
+* ``foo[3] ?? 'no'``
+* ``foo.baz ?? foo['baz'] ?? 'no'``
+
 Built-in Objects and Variables
 ------------------------------