test: add regression coverage for null metadata with empty state objects

[aws-kevinrickard]

Adds regression tests for the state/metadata consistency invariant that #213 fixes.

What this tests

After ShadowDocument.update(patch), both sides of the document must satisfy: if state.{desired,reported} is non-null, then metadata.{desired,reported} is non-null. A desired-side violation of this invariant causes getDelta() to throw NullPointerException in buildMetadata (one frame deep).

The root cause is JsonUtil.isMissing() treating empty objects ({}) as missing, which leads ShadowStateMetadata.merge() to remove metadata keys while the corresponding state nodes remain. When that removal empties the metadata tree, nullIfEmpty converts it to null.

Six cases cover the invariant from different angles. Case 5 exercises the same empty-object handling on the reported side, where it leaves the metadata tree inconsistent without surfacing as a crash — delta metadata is derived from the desired tree alone, so the reported-side violation is silent. Case 6 is the control (populated node, must pass everywhere).

Verification results

All assertions check the invariant (state non-null → metadata non-null) and safety (getDelta() does not throw). No assertions on metadata contents.

Case	Description	release_2.3.x	v2.3.13
1	empty object as only desired node	✓	✗
2	empty object beside a populated sibling	✓	✓
3	all children of a node removed by a null patch	✓	✗
4	empty object nested two levels down	✓	✓
5	empty object on the reported side	✓	✗
6	populated desired node (control)	✓	✓

release_2.3.x: 12 tests, 0 failures.
v2.3.13: 12 tests, 5 failures — the invariant assertion fails for cases 1, 3, and 5 (metadata null while state is non-null), and the getDelta() safety assertion additionally fails for cases 1 and 3 with the NPE from ShadowStateMetadata.buildMetadata. Case 5's violation is on the reported side, so it does not trigger the NPE.

The failures on the pre-fix tag confirm the tests are meaningful: they detect the condition #213 addresses.

Notes

• This PR targets main, which carries the [Fix] Leave empty objects in metadata #213 fix as of fix: leave empty objects in metadata (cherry-pick #213 to main) #225. The tests also pass on release_2.3.x (where [Fix] Leave empty objects in metadata #213 originally landed) and fail on any branch without it.
• Tests are in a new class (ShadowDocumentMetadataInvariantTest) rather than extending ShadowStateMetadataTest, because the latter differs between tags.
• The getDelta() safety assertion is a separate parameterized test method so failures report independently from the invariant assertion.

The base branch was changed.