[Serializer] Fix BackedEnumNormalizer behavior with partial denormalization

[yoeunes]

Q	A
Branch?	6.4
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Issues	Fixes #62313
License	MIT

This PR fixes a bug in BackedEnumNormalizer that occurs when using collect_denormalization_errors and allow_invalid_values at the same time.

When allow_invalid_values was set, the normalizer would return null for invalid data (like null) before NotNormalizableValueException could be thrown.

This caused the denormalization to fail with a fatal TypeError (e.g., Typed property ... must not be null) instead of populating a PartialDenormalizationException with the expected type errors.

This fix ensures that NotNormalizableValueException is still thrown when collect_denormalization_errors is active (not_normalizable_value_exceptions context key is set), allowing the Serializer to catch it and report the error correctly.

[nicolas-grekas]

Thank you @yoeunes.