[Concurrency] Make currentExecutor return a non-optional.

If we don't have any other executor to return, return the default
executor.

rdar://141348916

Author: al45tair

stdlib/public/Concurrency/Executor.swift

@@ -600,15 +600,15 @@ extension Task where Success == Never, Failure == Never {
   ///  If none of these exist, this property will be `nil`.
   @available(SwiftStdlib 6.2, *)
   @_unavailableInEmbedded
-  public static var currentExecutor: (any Executor)? {
+  public static var currentExecutor: any Executor {
     if let activeExecutor = unsafe _getActiveExecutor().asSerialExecutor() {
       return activeExecutor
     } else if let taskExecutor = unsafe _getPreferredTaskExecutor().asTaskExecutor() {
       return taskExecutor
     } else if let taskExecutor = unsafe _getCurrentTaskExecutor().asTaskExecutor() {
       return taskExecutor
     }
-    return nil
+    return defaultExecutor
   }
 
   /// Get the preferred executor for the current `Task`, if any.