Add fallback background color on iOS 13

lib/ios/ReactNativeNavigation.m

 	if (@available(iOS 13.0, *)) {
 #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 130000
 		keyWindow.backgroundColor = [UIColor systemBackgroundColor];
+#else
+		keyWindow.backgroundColor = [UIColor whiteColor];
 #endif
 	} else {
 		keyWindow.backgroundColor = [UIColor whiteColor];