Allow subjects valueUri to be null

[richardhallett]

Allow json schema validation to accept null values for valueUri

Purpose

Fix for datacite/datacite#1971

Approach

Ensure null can be an accepted validation type for json schema.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• New feature (non-breaking change which adds functionality)

• Breaking change (fix or feature that would cause existing functionality to change)

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