Need to send empty arrays for MultiPart requests

[Windisch94]

Its nearly the same as described in #599, but for arrays instead of strings.
When sending an empty array via @MultiPart and @Part the generator skips the property and doesnt send it.

When using without MultiPart und using @Field annotation the empty array got sent.
Example:

@MultiPart()
 @POST('/test')
  Future<ResponseDto> createFile({
    @Part(name: 'data[]') required List<String> data,
    @Part(name: 'images[]') required List<MultipartFile> images,
  });

When sending data as empty array the data property not defined