minor #20147 [Serializer] add nullable format to defaultContext circularReference handler to prevent normalize error (eltharin)

This PR was submitted for the 7.2 branch but it was merged into the 6.4 branch instead.

Discussion
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[Serializer] add nullable format to defaultContext circularReference handler to prevent normalize error

add nullable $format parameter if this defaultContext is used with $serializer->normalize where $format can be nullable.

compress arguments with only seventh set.

Commits
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cba78d5 just nullable

Author: javiereguiluz

components/serializer.rst

@@ -1500,7 +1500,7 @@ having unique identifiers::
 
     $encoder = new JsonEncoder();
     $defaultContext = [
-        AbstractNormalizer::CIRCULAR_REFERENCE_HANDLER => function (object $object, string $format, array $context): string {
+        AbstractNormalizer::CIRCULAR_REFERENCE_HANDLER => function (object $object, ?string $format, array $context): string {
             return $object->getName();
         },
     ];