Add null annotation to Async.SwitchToContext (#18059)

Author: BoundedChenn31

docs/release-notes/.FSharp.Core/9.0.200.md

@@ -1,6 +1,7 @@
 ### Fixed
 
 * Fix exception on Post after MailboxProcessor was disposed ([Issue #17849](https://github.com/dotnet/fsharp/issues/17849), [PR #17922](https://github.com/dotnet/fsharp/pull/17922))
+* Fix missing null annotation in Async.SwitchToContext ([Issue #18055](https://github.com/dotnet/fsharp/issues/18055), [PR #18059](https://github.com/dotnet/fsharp/pull/18059))
 
 ### Added
 

src/FSharp.Core/async.fs

@@ -868,11 +868,7 @@ module AsyncPrimitives =
 
     ///   - Initial cancellation check
     ///   - Call syncCtxt.Post with exception protection. This may fail as it is arbitrary user code
-#if BUILDING_WITH_LKG || NO_NULLCHECKING_LIB_SUPPORT
     let CreateSwitchToAsync (syncCtxt: SynchronizationContext) =
-#else
-    let CreateSwitchToAsync (syncCtxt: SynchronizationContext | null) =
-#endif
         MakeAsyncWithCancelCheck(fun ctxt -> ctxt.PostWithTrampoline syncCtxt ctxt.cont)
 
     ///   - Initial cancellation check

src/FSharp.Core/async.fsi

@@ -647,7 +647,7 @@ namespace Microsoft.FSharp.Control
         /// <category index="4">Threads and Contexts</category>
         ///
         /// <example-tbd></example-tbd>
-        static member SwitchToContext :  syncContext:System.Threading.SynchronizationContext -> Async<unit> 
+        static member SwitchToContext : syncContext:(System.Threading.SynchronizationContext | null) -> Async<unit> 
 
         /// <summary>Creates an asynchronous computation that captures the current
         /// success, exception and cancellation continuations. The callback must