fix: only stringify present tags

fix #4

Author: Midnighter

src/structurizr/model/model_item.py

@@ -55,7 +55,8 @@ def split_tags(cls, tags: Union[str, List[str]]) -> List[str]:
     def dict(self, **kwargs) -> dict:
         """"""
         obj = super().dict(**kwargs)
-        obj["tags"] = ",".join(obj["tags"])
+        if "tags" in obj and len(obj["tags"]) > 0:
+            obj["tags"] = ",".join(obj["tags"])
         return obj