Don't cause index out of range exn in completion

[brianrourkeboll]

Description

Fixes #17226.

• Dotting into delegates with multiple type arguments would trigger an index out of range exception. Now it doesn't.

Checklist

• Tests: I spent a while trying to get a test for this to work, but it seems like it might be rather difficult, since this pathway only seems to be called sometimes. Compare: https://github.com/dotnet/fsharp/blob/ef2307d18b2d97312680187b3d189a0076fdaa94/src/Compiler/Service/FSharpCheckerResults.fs#L1316-L1318

  But here's a recording of the fix working:

  https://github.com/dotnet/fsharp/assets/14795984/977083b5-5018-422a-bb54-ba87d5779bbe

• [x] Release notes entry updated.

[github-actions[bot]]

:heavy_exclamation_mark: Release notes required

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:white_check_mark: Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.400.md