Change: URI result field defaults now to an empty string instead of None

None was later serialized and sent as None string in the URI result field.
gvmd will ignore/hide the URI field if it is an empty string.

notus/scanner/messages/result.py

@@ -28,7 +28,7 @@ def __init__(
         oid: str,
         value: str,
         port: str = "package",
-        uri: str = None,
+        uri: str = "",
         result_type: ResultType = ResultType.ALARM,
         message_id: Optional[UUID] = None,
         group_id: Optional[UUID] = None,