fix: Empty ObjectNode type safety

[CraigMacomber]

Description

ObjectNodes with no fields will now emit a compiler error if constructed from an object literal with fields.
This matches the behavior of non-empty ObjectNodes which already gave errors when unexpected properties were provided.

Also adds an "unsound union properties" test demoing a type soundness issue which impacts tree but is a limitation of TypeScript which Tree can no workaround.

Breaking Changes

Any code which constructed object nodes with no fields from object literals with fields should have those fields removed as they have no impact at runtime.

Reviewer Guidance

The review process is outlined on this wiki page.

[msfluid-bot]

Code Coverage Summary

No packages impacted by the change.

---

Baseline commit: 9a8f69a
Baseline build: 308718
Happy Coding!!

Code coverage comparison check passed!!

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +245 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	467.24 KB	467.27 KB	⯅ +35 Bytes
azureClient.js	564.01 KB	564.06 KB	⯅ +49 Bytes
connectionState.js	724 Bytes	724 Bytes	■ No change
containerRuntime.js	263.43 KB	263.45 KB	⯅ +14 Bytes
fluidFramework.js	428.31 KB	428.32 KB	⯅ +14 Bytes
loader.js	134.18 KB	134.19 KB	⯅ +14 Bytes
map.js	42.71 KB	42.71 KB	⯅ +7 Bytes
matrix.js	150.15 KB	150.16 KB	⯅ +7 Bytes
odspClient.js	529.85 KB	529.89 KB	⯅ +49 Bytes
odspDriver.js	97.88 KB	97.9 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.81 KB	42.83 KB	⯅ +14 Bytes
sharedString.js	166.23 KB	166.24 KB	⯅ +7 Bytes
sharedTree.js	418.76 KB	418.77 KB	⯅ +7 Bytes
Total Size	3.38 MB	3.38 MB	⯅ +245 Bytes

---

Baseline commit: 9a8f69a

Generated by 🚫 dangerJS against 0631ae5

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> [email protected] ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> [email protected] serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> [email protected] check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  170005 links
    1595 destination URLs
    1825 URLs ignored
       0 warnings
       0 errors

[Josmithr]

Left a couple of questions but overall, this looks like a great safety improvement!