fix: tags always need to be a string

Author: Midnighter

src/structurizr/model/model_item.py

@@ -55,7 +55,7 @@ def split_tags(cls, tags: Union[str, List[str]]) -> List[str]:
     def dict(self, **kwargs) -> dict:
         """"""
         obj = super().dict(**kwargs)
-        if "tags" in obj and len(obj["tags"]) > 0:
+        if "tags" in obj:
             obj["tags"] = ",".join(obj["tags"])
         return obj