Use null-aware elements in dev/devicelab/lib/integration_tests.dart

dev/devicelab/lib/tasks/integration_tests.dart

@@ -295,9 +295,9 @@ class DriverTest {
       // Make the device ID available in the driver code, so tools like ADB can
       // reference it if needed.
       final env = <String, String>{
-        if (environment != null) ...environment!,
+        ...?environment,
         'FLUTTER_DEVICE_ID_NUMBER': deviceId,
-        if (devicelabAdbPath != null) 'FLUTTER_ADB_PATH': devicelabAdbPath,
+        'FLUTTER_ADB_PATH': ?devicelabAdbPath,
       };
 
       final options = <String>[