Cmds.py: Emit usage message by default

Running an application derived from Cmds throws an error if invoked
without arguments, fix that.

Signed-off-by: Jan Lindemann <jan@janware.com>

tools/python/jwutils/Cmds.py

@@ -92,6 +92,19 @@ class Cmds: # export
         args = self.__parser.parse_args()
         set_flags(args.log_flags)
         set_level(args.log_level)
+
+        # This is the toplevel parser, i.e. no func member has been added to the args via
+        #
+        #    Cmds.__init__()
+        #    Cmds.__add_cmd_to_parser(cmd, subparsers)
+        #    CmdXXX.add_parser(parsers)
+        #    super().add_parser(parsers)
+        #    Cmd.__parser.set_defaults(func=self.run)
+        #
+        if not hasattr(args, 'func'):
+            self.__parser.print_help()
+            return None
+
         return await args.func(args)
 
     def __del__(self):