Fix issue with cancelling AsyncBatchingWorkQueue

This was simply a mistake on my part. We should not assume that code is unreachable when it is reached because an OperationCanceledException is caught.

src/Razor/src/Microsoft.AspNetCore.Razor.ProjectEngineHost/Utilities/AsyncBatchingWorkQueue`2.cs

@@ -271,9 +271,8 @@ void AddItemsToBatch(IEnumerable<TItem> items)
         catch (OperationCanceledException)
         {
             // Silently continue to allow the next batch to be processed.
+            return default;
         }
-
-        return Assumed.Unreachable<TResult?>();
     }
 
     private (ImmutableArray<TItem> items, CancellationToken batchCancellationToken) GetNextBatchAndResetQueue()