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

[srawlins]

This is cleanup work required to start enforcing this with static analysis, as per dart-lang/sdk#53253.

Real quick this issue is that this code is unsafe:

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

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