Drop checks for unsound null values (#189)

Towards #111

During the null safety migration it was possible for
`StreamSubscription.cancel()` to return `null` in some cases. Now that
this package is only used with sound null safe code it isn't possible
for these nulls to flow through anymore so it is not necessary to filter
for them.

lib/src/aggregate_sample.dart

@@ -135,8 +135,6 @@ extension AggregateSample<T> on Stream<T> {
         } else {
           triggerSub!.pause();
         }
-        // Handle opt-out nulls
-        cancels.removeWhere((Object? f) => f == null);
         if (cancels.isEmpty) return null;
         return cancels.wait.then((_) => null);
       };

lib/src/async_expand.dart

@@ -78,10 +78,7 @@ extension AsyncExpand<T> on Stream<T> {
       }
       controller.onCancel = () {
         if (subscriptions.isEmpty) return null;
-        var cancels = [for (var s in subscriptions) s.cancel()]
-          // Handle opt-out nulls
-          ..removeWhere((Object? f) => f == null);
-        return cancels.wait.then((_) => null);
+        return [for (var s in subscriptions) s.cancel()].wait.then((_) => null);
       };
     };
     return controller.stream;

lib/src/combine_latest.dart

@@ -128,9 +128,7 @@ extension CombineLatest<T> on Stream<T> {
         var cancels = [
           sourceSubscription!.cancel(),
           otherSubscription!.cancel()
-        ]
-          // Handle opt-out nulls
-          ..removeWhere((Object? f) => f == null);
+        ];
         sourceSubscription = null;
         otherSubscription = null;
         return cancels.wait.then((_) => null);
@@ -230,11 +228,7 @@ extension CombineLatest<T> on Stream<T> {
       }
       controller.onCancel = () {
         if (subscriptions.isEmpty) return null;
-        var cancels = [for (var s in subscriptions) s.cancel()]
-          // Handle opt-out nulls
-          ..removeWhere((Object? f) => f == null);
-        if (cancels.isEmpty) return null;
-        return cancels.wait.then((_) => null);
+        return [for (var s in subscriptions) s.cancel()].wait.then((_) => null);
       };
     };
     return controller.stream;

lib/src/merge.dart

@@ -90,11 +90,7 @@ extension Merge<T> on Stream<T> {
       }
       controller.onCancel = () {
         if (subscriptions.isEmpty) return null;
-        var cancels = [for (var s in subscriptions) s.cancel()]
-          // Handle opt-out nulls
-          ..removeWhere((Object? f) => f == null);
-        if (cancels.isEmpty) return null;
-        return cancels.wait.then((_) => null);
+        return [for (var s in subscriptions) s.cancel()].wait.then((_) => null);
       };
     };
     return controller.stream;

lib/src/switch.dart

@@ -124,9 +124,7 @@ extension SwitchLatest<T> on Stream<Stream<T>> {
         var cancels = [
           if (!outerStreamDone) outerSubscription.cancel(),
           if (sub != null) sub.cancel(),
-        ]
-          // Handle opt-out nulls
-          ..removeWhere((Object? f) => f == null);
+        ];
         if (cancels.isEmpty) return null;
         return cancels.wait.then(_ignore);
       };