feat(nns): Support SelfDescribingValue::Null

[jasonz-dfinity]

Why

To make "empty" values more explicit, we opt to support a Null variant to represent absent values.

The downside of diverging from ICRC3 was considered. However, if we choose to adopt ICRC3 for proposals in the future (if at all), it will likely affect the entire method (list_proposals) rather than the "action" part of the proposal.

What

• Introduce a Null variant for storage and api
• Define conversions between them
• Define impl From<Option<T>> for SelfDescribingValue using Null for None
• Remove add_field_with_empty_as_fallback as null can be used to represent the "emptiness"
• Use null for some ManageNeuron commands where no content is needed

[github-actions]

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No canister behavior changes - behind feature flag