Return completed task instead of null

Proprietary

Replace return null in Task-returning methods with return Task.CompletedTask.

OpenRewrite.Recipes.CSharp.CodeQuality.Performance.ReturnCompletedTask

mod run . --recipe=OpenRewrite.Recipes.CSharp.CodeQuality.Performance.ReturnCompletedTask

Install it once, then run it on every repository under the current directory. New to the Moderne CLI? Set it up.