LPS-41187 Use Validator instead of null check so that the taglibs also consider an empty string as no tags/categories are selected
LPS-41187 The structureId was being included twice when toLanguageId is null (see line 215)
LPS-41187 EditArticleAction expects the parameter structureId so we need to provide that one.

I think we should only use ddmStructureId everywhere. It seems to me that the refactoring to use DDM was not 100% finished in little details like this, but I decided to not try to fix that for this bug since it would create unnecessary risk at this point.