Sniff::is_validated(): recognize null coalesce (equal) operator as a way to validate a variable
Sniff::is_validated(): recognize null coalesce (equal) operator as a way to validate a variable

This adds recognition of the coalesce operator `??` (PHP 7.0) and the coalesce equals operator `??=`, as will be added in PHP 7.4, to the `Sniff::is_validated()` method.

This prevents false positives where variables would be seen as "not validated", when the variable has in fact been validated via a coalesce equals assignment in a previous statement.

Related to 764, 840

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