Fix for bug when attempting to Cursor Slice an empty result set

cajuncoding / GraphQL.RepoDB

A set of extensions for working with HotChocolate GraphQL and Database access with micro-orms such as RepoDb (or Dapper). This extension pack provides access to key elements such as Selections/Projections, Sort arguments, & Paging arguments in a significantly simplified facade so this logic can be leveraged in the Serivces/Repositories that encapsulate all data access (without dependency on IQueryable and execution outside of the devs control).

MIT License

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When no results are returned by an underlying data source (e.g. empty list/table, etc) IEnumerableCursorPagingCustomExtensions.SliceAsCursorPage() creates a new CursorPageSlice and passes null for the results parameter. This causes an ArgumentException to be thrown by the CursorPageSlice constructor.

This bug was discovered when testing a resolver over a postgresql table I had just created but contained no rows. The GraphQL query should return an empty node collection and totalCount = 0.

Relevant line for the bug:

https://github.com/cajuncoding/GraphQL.RepoDB/blob/52dabefdeec5f6d29e4f97c3a927bc05426ac31c/GraphQL.PreProcessingExtensions/Paging/CursorPaging/IEnumerableCursorPagingCustomExtensions.cs#L45

This can be reproduced by modifying Tests.StarWarsCharacterResolver.CreateCharacters to return an empty list and running the TestParamsContextCursorPagingOnlyFirst test.

This PR modifies behavior this by:

Avoid throwing ArgumentException in CursorPageSlice to match the way Offset pagination handles empty results.
Swapped from creating a new List to Enumerable.Empty() to avoid un-needed allocation.

cajuncoding / GraphQL.RepoDB

Fix for bug when attempting to Cursor Slice an empty result set #7