Fix handling of empty AggregateExn

Author: gusty

src/FSharpPlus/Extensions/Extensions.fs

@@ -132,7 +132,10 @@ module Extensions =
                 computation,
                 ts.SetResult,
                 (function
-                    | :? AggregateException as agg -> ts.SetException agg.InnerExceptions
+                    | :? AggregateException as aex ->
+                        let exns = aex.InnerExceptions
+                        if exns.Count = 0 then ts.SetException aex
+                        else ts.SetException exns
                     | exn -> ts.SetException exn),
                 (fun _ -> ts.SetCanceled ()),
                 cancellationToken)