fix: admit empty-object allOf members instead of crashing

[nottmey]

Summary

allOf: [{$ref: JsonObject}, {type: object, properties: {...}}] crashes with FormatException: allOf only supports objects because JsonObject (an open empty object: type: object, properties: {}, additionalProperties: {}) resolves to ResolvedEmptyObject, not ResolvedObject, and the resolver rejected any non-ResolvedObject allOf member.

Fix

Allow ResolvedEmptyObject as an allOf member (alongside ResolvedObject) in the type-check at resolver.dart. An empty open object intersects trivially — every JSON object satisfies it — so it is a valid allOf member that simply contributes no properties to the merged object the renderer builds.

The render pass already only merges RenderObject members (case ResolvedAllOf in render_tree.dart), so a ResolvedEmptyObject member is already a no-op there; this just makes the resolver stop crashing on the way there. The allOf is preserved (not collapsed), so the parent schema keeps its name — important for test emission and downstream naming.

This mirrors the spirit of PR #138's comment_references handling: spec prose / spec shapes that trip Dart's tooling should be sanitized at the generator boundary rather than crashing.

Spec that surfaced this

Backstage's catalog OpenAPI spec uses this exact shape for RecursivePartialEntityMeta:

https://github.com/backstage/backstage/blob/master/plugins/catalog-backend/src/schema/openapi.yaml

Schema excerpt:

JsonObject:
  type: object
  properties: {}
  additionalProperties: {}
  description: A type representing all allowed JSON object values.
RecursivePartialEntityMeta:
  allOf:
    - $ref: '#/components/schemas/JsonObject'
    - type: object
      properties: { ... }
      additionalProperties: false

With this fix, space_gen generates the full Backstage catalog client from the remote spec with no patches or vendoring required.

Test plan

• Added allOf admits empty-object members to test/resolver_test.dart — verifies an allOf containing an open empty object plus a real object resolves to a ResolvedAllOf with both members preserved (no crash, no collapse).
• dart test test/resolver_test.dart — all resolver tests pass.
• dart test (full suite) — 548 tests pass, no regressions.
• End-to-end: ran this branch's bin/space_gen.dart against the remote Backstage catalog spec URL; full client generated successfully (name: devhub_client, all lib/ emitted, RecursivePartialEntityMeta model present, 138 generated round-trip tests pass).