Return null instead of empty in Future.catchError callbacks (#72409)

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -618,7 +618,7 @@ class BrowserManager {
       throwToolExit('${runtime.name} exited with code $browserExitCode before connecting.');
     }).catchError((dynamic error, StackTrace stackTrace) {
       if (completer.isCompleted) {
-        return;
+        return null;
       }
       completer.completeError(error, stackTrace);
     }));
@@ -630,7 +630,7 @@ class BrowserManager {
     }).catchError((dynamic error, StackTrace stackTrace) {
       chrome.close();
       if (completer.isCompleted) {
-        return;
+        return null;
       }
       completer.completeError(error, stackTrace);
     }));

packages/flutter_tools/test/integration.shard/test_driver.dart

@@ -179,7 +179,10 @@ abstract class FlutterTestDriver {
     // it forcefully and it won't terminate child processes, so we need to ensure
     // it's running before terminating.
     await resume().timeout(defaultTimeout)
-        .catchError((Object e) => _debugPrint('Ignoring failure to resume during shutdown'));
+        .catchError((Object e) {
+          _debugPrint('Ignoring failure to resume during shutdown');
+          return null;
+        });
 
     _debugPrint('Sending SIGTERM to $_processPid..');
     io.Process.killPid(_processPid, io.ProcessSignal.sigterm);