undefined or nil Args default to ArbitraryArgs (#1612)

Author: umarcor

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)
 }

user_guide.md

@@ -326,8 +326,8 @@ In both of these cases:
 
 ## Positional and Custom Arguments
 
-Validation of positional arguments can be specified using the `Args` field
-of `Command`.
+Validation of positional arguments can be specified using the `Args` field of `Command`.
+If `Args` is undefined or `nil`, it defaults to `ArbitraryArgs`.
 
 The following validators are built in: