undefined or nil Args default to ArbitraryArgs (#1612)

command.go

@@ -1012,7 +1012,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 
 func (c *Command) ValidateArgs(args []string) error {
 	if c.Args == nil {
-		return nil
+		return ArbitraryArgs(c, args)
 	}
 	return c.Args(c, args)
 }