Add function tests

lib/configuration_provider.dart

@@ -1,4 +1,4 @@
 abstract class ConfigurationProvider {
   bool containsKey(String key);
   String? getOrNull(String key, {String? defaultValue});
-}
+}

lib/system_config.dart

@@ -21,6 +21,6 @@ class SystemConfig {
   }
 
   static String? getOrNull(String key, {String? defaultValue}) {
-    return getProvider().getOrNull(key);
+    return getProvider().getOrNull(key, defaultValue: defaultValue);
   }
 }