Avoid passing a nullable value to Completer<nn-type>.complete

This is cleanup work required to start enforcing this with static
analysis, as per #53253.

Real quick this issue is that this code is unsafe:

```dart
void f(Completer<int> c, int? i) {
  Future<int>.value(i); // Ouch!
  c.complete(i);        // Ouch!
}
```

Change-Id: I12c4f0a92a2071fb47759d9626689e00cfa42f8d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324763
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

pkg/analysis_server/test/stress/utilities/server.dart

@@ -75,8 +75,9 @@ class RequestData {
 
   /// Return a future that will complete when a response is received.
   Future<Response> get respondedTo {
-    if (_response != null) {
-      return Future.value(_response);
+    final response = _response;
+    if (response != null) {
+      return Future.value(response);
     }
     var completer = _responseCompleter ??= Completer<Response>();
     return completer.future;