Nullable QueryMap fails to compile

[techouse]

Hi, I really love your package, however, I came across an issue where I wanted to pass a QueryMap as an optional/nullable value and this resulted in it simply being added to the list of $param using $params.addAll(theNullableQueryMap). This however fails, because a nullable value can not be added to a Map using Map.addAll.

Essentially this:

@Get(path: 'articles')
Future<Response<List<Article>>> getArticles({
  @Query('article_id') String? articleId,
  @Query('author_name') String? authorName,
  @Query('limit') int? limit = 10,
  @QueryMap() Map<String, dynamic>? filters,
});

results in

@override
Future<Response<List<Article>>> getArticles(
    {String? articleId,
    String? authorName,
    int? limit = 10,
    Map<String, dynamic>? filters}) {
  final $url = 'articles';
  final $params = <String, dynamic>{
    'article_id': articleId,
    'authorName': authorName,
    'limit': limit
  };
  $params.addAll(filters); // <-- this here will fail to compile because `filters` can't be nullable 
  final $request = Request('GET', $url, client.baseUrl, parameters: $params);
  return client.send<List<Article>, Article>($request);
}

It throws this compilation error:

Error: The argument type 'Map<String, dynamic>?' can't be assigned to the parameter type 'Map<String, dynamic>' because 'Map<String, dynamic>?' is nullable and 'Map<String, dynamic>' isn't.

- 'Map' is from 'dart:core'.
   $params.addAll(filters);
                  ^

I would propose it checks if filters is null before using $params.addAll

final $params = <String, dynamic>{
  'article_id': articleId,
  'authorName': authorName,
  'limit': limit
};
if (filters != null) {
  $params.addAll(filters);
}

or simply using something more elegant like:

final $params = <String, dynamic>{
  'article_id': articleId,
  'authorName': authorName,
  'limit': limit,
  ...?filters,
};