Fixed altStart and altEnd not being provided causing KeyError

src/pyrmv/classes/message.py

@@ -81,8 +81,10 @@ class Message:
                 f"{data['validToDate']} {data['validToTime']}", "%Y-%m-%d %H:%M:%S"
             )
         )
-        self.date_start_alt: str = data["altStart"]
-        self.date_end_alt: str = data["altEnd"]
+        self.date_start_alt: Union[str, None] = (
+            None if "altStart" not in data else data["altStart"]
+        )
+        self.date_end_alt: Union[str, None] = None if "altEnd" not in data else data["altEnd"]
         self.time_modified: Union[datetime, None] = (
             None
             if "modDate" not in data or "modTime" not in data