Remove "value" as required element of address_level

[jwass]

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1. Cosmetic change.
2. Documentation change by member.
3. Documentation change by Overture tech writer.
4. Material change.

Description

This fixes a bug in the schema where unknown address_levels were disallowed. Allow them by still requiring the
addressLevelContainer struct, but make the "value" field optional.

Reference

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1. TODO.

Testing

jv

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1. Add relevant examples.
2. Add relevant counterexamples.
3. Update any counterexamples that became obsolete. For example, if a counterexample uses property A but is not intended to test property A's validity, and you made a schema change that invalidates property A in that counterexample, fix the counterexample to align it with your schema change.
4. Update in-schema documentation using plain English written in complete sentences, if an update is required.
5. Update Docusaurus documentation, if an update is required.
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[vcschapp]

Note to myself: We had meant to allow null but never did. Now using {} placeholder instead of allowing null.

[vcschapp]

Note to myself: We had meant to allow null but never did. Now using {} placeholder instead of allowing null.

Reason for {} is that arrays with explicit null value are somewhat uncommon/surprising. We feel that an existing object/structure with missing fields is a more common/less surprising data structure.

Agreed 2024-10-02 Schema TF meeting: We'll follow this approach and try to avoid explicit null value in array.

There's an open point about whether address_levels should be empty if it has no meaningful content. Punting on that.