TheCapabilityParser Interface should infer nb is optional if all its properties are optional

[vasco-santos]

While working on storacha/w3up#184 to add nb properties to store/list and upload/list to support pagination, got to this problem.

When we have nb with all optional properties like:

 nb: {
      /**
       * Item where a previous list operation stopped, inclusive of the previous
       * result set. Use this value to start a new operation, inorder to paginate
       * list.
       */
      cursor: Schema.string().optional(),
      /**
       * Size of the page being requested to list.
       */
      size: Schema.integer().optional(),
    }

and we want to invoke like:

const delegation = Upload.list.invoke({
      issuer: alice,
      audience: bob,
      with: account.did(),
      proofs: [await any],
 })

the interface types fail with Property 'nb' is missing in type. If we just add an empty object for nb there types get happy.

I looked through the types in https://github.com/web3-storage/ucanto/blob/5d04d169d09bd691261ddc86449488c05b093683/packages/interface/src/capability.ts#L178 but could not get to a solution to create PR. Main motivation here is to not require a breaking change to add new optional nb. However, it also makes sense in general to not need to provide empty object if everything is optional