Fix AnyType serialization with empty-string dictionary keys

[glen-84]

Summary

• ResultDocument.WriteDataCore early-returns on a zero-length span, fixing an ArgumentOutOfRangeException when writing an empty property name.
• ResultDocument.ReadLocalData early-returns on a zero-length read, so properties with SizeOrLength == 0 whose Location was never backed by a rented chunk (e.g. { "": null } with no other local data) don't trip _data[startChunkIndex].
• Removes the propertyName.Length == 0 throws from both ResultElement.SetPropertyName overloads — empty strings are valid JSON property names and the previous guard was masking the writer bug above.
• Adds an AnyType regression test covering a resolver returning a Dictionary<string, object?> with an empty-string key and a null value (the minimal case that exercises both the zero-length write and zero-length read paths).

Test plan

• dotnet test src/HotChocolate/Core/test/Types.Tests/HotChocolate.Types.Tests.csproj --filter "FullyQualifiedName~AnyTypeTests.Output_Return_Dictionary_With_Empty_Key" — green on net9.0.
• dotnet test src/HotChocolate/Core/test/Types.Tests/HotChocolate.Types.Tests.csproj --filter "FullyQualifiedName~AnyType" — full AnyType suite 75/75 on net9.0.

Closes #9774

[Copilot]

Pull request overview

This PR fixes AnyType JSON serialization for dictionaries that contain an empty-string key by allowing empty property names to flow through ResultElement and preventing a writer crash when attempting to write a zero-length byte span.

Changes:

• Allow empty-string JSON property names by removing the Length == 0 exception guards in ResultElement.SetPropertyName overloads.
• Add an early-return in ResultDocument.WriteDataCore for data.Length == 0 to avoid exceptions when writing empty property names.
• Add a regression test validating output when a resolver returns a dictionary with an empty-string key.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/HotChocolate/Core/test/Types.Tests/Types/Scalars/AnyTypeTests.cs	Adds a regression test for empty-string dictionary keys in AnyType output.
src/HotChocolate/Core/src/Types/Text/Json/ResultElement.cs	Removes empty-name guards so empty JSON property names are permitted.
src/HotChocolate/Core/src/Types/Text/Json/ResultDocument.cs	Avoids exceptions by no-op’ing WriteDataCore on zero-length writes.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.