Return 200 empty payload for cached reads with empty result

[Souvik Ghosh (souvikghosh04)]

Summary

Fixes #3704 (and its duplicate #3446).

When entity/runtime caching is enabled and a REST/GraphQL read returns zero
rows, DAB returned HTTP 500 (System.InvalidOperationException) during
response serialization. With caching disabled, the identical request correctly
returns 200 with an empty payload (e.g. {"value":[]}).

Root cause

In the cache read path,
SqlQueryEngine.GetResultInCacheScenario → ParseResultIntoJsonDocument(JsonElement? result):

• For an empty result set, the executor's GetJsonResultAsync<JsonElement> yields
  default(JsonElement) — a struct whose ValueKind is JsonValueKind.Undefined
  and which has no backing JsonDocument. Because it is a value type, it is not
  null, so the nullable result has a value.
• ParseResultIntoJsonDocument then called
  JsonSerializer.SerializeToUtf8Bytes(result), which invokes
  JsonElement.WriteTo → CheckValidInstance() and throws
  InvalidOperationException, surfaced as HTTP 500.

The non-cached path never hits this: it returns a null JsonDocument? for an
empty result, which downstream renders as an empty payload. The cache path was
missing that empty guard. The failure occurs on both cache miss and cache hit,
and applies to list (FOR JSON PATH) and by-PK reads, for both REST and
GraphQL (this method is shared).

Fix

Guard the Undefined case in ParseResultIntoJsonDocument and return null,
matching the non-cached empty-result path:

if (result is { ValueKind: JsonValueKind.Undefined })
{
    return null;
}

The pre-existing null-nullable behavior (serializing to a JSON null document)
is intentionally left unchanged, since only the Undefined element throws.

Tests

• Added SqlQueryEngineHelperTests.ParseResultIntoJsonDocument_UndefinedElement_ReturnsNull,
  which invokes the method with default(JsonElement) and asserts it returns
  null (previously it threw InvalidOperationException).
• The existing ParseResultIntoJsonDocument_HandlesValuesAndNull cases continue
  to pass unchanged.
• All 8 SqlQueryEngineHelperTests pass; dotnet format --verify-no-changes is
  clean on the changed files.

[Copilot]

Copilot review overview

🟢 Approval recommended

The fix matches existing non-cached empty-result behavior and has focused regression coverage.

Review effort: Lite
Findings: None

What changed in this PR

Fixes cached empty SQL reads that previously caused HTTP 500 by handling undefined JsonElement values before serialization.

Changes:

• Return null for undefined cached results.
• Add a regression unit test.

File	Description
src/​Core/​Resolvers/​SqlQueryEngine.cs	Guards undefined cached results.
src/​Service.Tests/​UnitTests/​SqlQueryEngineHelperTests.cs	Tests undefined-element handling.

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