[Serializer] Fix passing null to str_contains()

Author: Erwin Dirks

Normalizer/AbstractObjectNormalizer.php

@@ -472,7 +472,7 @@ private function validateAndDenormalize(string $currentClass, string $attribute,
             // PHP's json_decode automatically converts Numbers without a decimal part to integers.
             // To circumvent this behavior, integers are converted to floats when denormalizing JSON based formats and when
             // a float is expected.
-            if (Type::BUILTIN_TYPE_FLOAT === $builtinType && \is_int($data) && str_contains($format, JsonEncoder::FORMAT)) {
+            if (Type::BUILTIN_TYPE_FLOAT === $builtinType && \is_int($data) && null !== $format && str_contains($format, JsonEncoder::FORMAT)) {
                 return (float) $data;
             }