Completion fails after the first argument for programs without sub-commands

[marckhouzam]

Programs that don't have sub-commands can accept any number of arguments.
However, when doing shell completion for such programs, within the __complete code this very __complete command makes it that the program suddenly has a sub-command, and the call to Find() -> legacyArgs() will then return an error if there are more than one argument on the command-line being completed.

This only happens when the program does not specify any value for its Args field, case which leads to legacyArgs() being called.

Here is an illustration. The program used follows.

# prog has no sub-commands
bash-5.1$ ./prog -h
Usage:
  prog [flags]

Flags:
  -h, --help   help for prog

# prog has a ValidArgsFunction to complete arguments
bash-5.1$ ./prog __complete ''
arg1
arg2
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

# Completion fails to continue completing arguments
bash-5.1$ ./prog __complete arg1 ''
[Debug] [Error] Unable to find a command for arguments: [arg1]
:0
Completion ended with directive: ShellCompDirectiveDefault

Test program

package main

import (
	"fmt"

	"github.com/spf13/cobra"
)

var rootCmd = &cobra.Command{
	Use: "prog",
	ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
		return []string{"arg1", "arg2"}, cobra.ShellCompDirectiveNoFileComp
	},
        Args: nil,
	Run: func(cmd *cobra.Command, args []string) {
		fmt.Println(fmt.Sprintf("rootCmd called with args %v", args))
	},
}
func main() {
	if err := rootCmd.Execute(); err != nil {
		fmt.Println("Got error", err.Error())
	}
}