Update FieldDefinitionNormalizer.php to allow NULL values in JSON:API schema.

Provided patch in issue was outdated and not working anymore. The change adds the "x-nullable" attribute to a field's schema if it is an optional value.

added 6 commits

• 6ca1370a...3eec8b01 - 5 commits from branch project:8.x-1.x
• 4ba37996 - Merge branch schemata:8.x-1.x into 8.x-1.x

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added 3 commits

• 4ba37996...9d41c1af - 2 commits from branch project:8.x-1.x
• 5ccb0b2f - Update FieldDefinitionNormalizer.php to allow NULL values in JSON:API schema.

Compare with previous version

merged