Completion: fix completion in empty dot lambda prefix

[auduchinok]

Fixes code completion on an empty dot lambda prefix:

[""] |> List.map _.{caret}

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.300.md

[psfinaki]

Thanks!

[vzarytovskii]

We should have a release note for that one probably, since it's a fix for some tooling behaviour