Fix index out of range panic when Run is given no arguments

[mrueg]

Command.Run indexes into the argument slice in two places without first checking that it is non-empty, so passing an empty slice panics instead of returning.

setupDefaults

Reads osArgs[0] to derive a default command name:

cmd := &cli.Command{Action: func(context.Context, *cli.Command) error { return nil }}
cmd.Run(context.Background(), []string{})
// panic: runtime error: index out of range [0] with length 0

Affects any root command that does not set Name explicitly.

checkShellCompleteFlag

Reads arguments[len(arguments)-1]:

cmd := &cli.Command{Name: "x", EnableShellCompletion: true}
cmd.Run(context.Background(), nil)
// panic: runtime error: index out of range [-1]

Affects any command with EnableShellCompletion set, named or not.

Fix

Guard both index accesses. A command invoked with no arguments now runs its Action as it would with any other argument list, and simply keeps an empty Name when there is no argv[0] to derive one from.

This is a pure robustness fix: in practice callers pass os.Args, which is never empty, so no working behaviour changes — the only reachable states previously were the two panics. It matters mostly for tests and for library consumers that build an argument slice programmatically.

Testing

Adds TestRunWithNoOsArgs (named / shell-completion-enabled / unnamed) and a no arguments at all case to the existing Test_checkShellCompleteFlag table. Both fail on main without the change.

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