Raise runtime error for unknow flavor

* fixes passing of None value to non-noneable parameters in filter
function

Author: ByteOtter

src/gardenlinux/features/__main__.py

@@ -118,6 +118,9 @@ def main() -> None:
 
     additional_filter_func = lambda node: node not in args.ignore
 
+    if flavor is None or "":
+        raise RuntimeError("Flavor could not be determined")
+
     if args.type == "arch":
         print(arch)
     elif args.type in ("cname_base", "cname", "graph"):